CHAPTER II

MORPHOLOGY AND STRUCTURE OF SHELL

Page
Appearance and principal axes________________________________________ 16 The oyster is a nearly bilaterally symmetrical
Dlmenslons_ ___ __ __ _ __ __ __ __ __ __ __ __ _ 20
Shape of shells____ __ __ __ __ __ __ __ __ __ __ __ _ 21
mollusk with the plane of symmetry passing be-
Growth rings and growth radIL______________________________________ 26 tween the two valves parallel to their surfaces.
Changes In the direction of principal axes of sheIL____________________ 27 In orienting any bivalve it is customary to hold
Dimensional relationships of shelL _ 29
Shell area__________ __ __ __ __ __ __ __ __ __ __ __ 30 it vertically with the narrow side uppermost (fig.
Chalky deposits _ __ __ __ __ __ __ __ __ __ __ __ __ __ 32 15). The narrow end or apex of the shell is called
Chambering and bllsters__ 35
Structure of shelL _ __ __ __ __ __ __ __ __ 36 the umbo (plural, umbos or umbones) or beak.
Organic material of the sheIL________________________________________
Muscle attachment._ __ __ __ __ __ __ __ __ __ __
37
41
A band of horny and elastic material, the ligament
Chemical composltlon________________________________________________ 43 (fig. 16) joins the valves at the hinge on which
Bibliography ___ __ __ __ __ __ ___ __ __ _ __ __ __ 45 they turn in opening or closing the shell.
In many bivalves the hinge carries a series of
APPEARANCE AND PRINCIPAL AXES interlocking teeth, but these structures are absent
The body of the oyster is covered with two in the family Ostreidae. The hinge consists of the
calcareous valves joined together by a resilient following parts: a projecting massive structure
ligament along the narrow hinge line. The valves within the right valve, the buttress, according to
are slightly asymmetrical. The left one is larger Stenzel's terminology, supports the midportion of
and deeper than the right one, which acts as a lid. the ligament and fits the depression on the left
Under normal conditions the oyster rests on the valve. The tract made by the buttress during
left valve or is cemented by its left valve to the the growth of the shell along the midportion of the
substratum. The difference between the right ligamental area is the resilifer. On the left valve
(flat) and left (cuplike) valve is to a certain degree the resilifer is the tract left on the depression.
common to all the species of oysters which have The central part of the ligament is called resilium.
been sufficiently studied. Orton's (1937) state- The pointed end of the valve or the beak repre-
ment with reference to Ostrea edulis that: "In sents the oldest part of a shell. In old individuals
life the flat or right valve usually rests on the sea it reaches considerable size (fig. 17). The beaks
bottom and is often referred to as the lower one" are usually curved and directed toward the
is an obvious oversight. posterior end of the mollusk although in some
In C. virginica the left valve is almost always specimens they may point toward the anterior.
thicker and heavier than the right one. When In the majority of bivalves other than oysters
oysters of this species are dumped from the deck the beaks usually point forward. The direction
of a boat and fall through water they come to rest and degree of curvature of the beaks of oysters as
on their left valves. I observed this many times well as their relative proportions vary greatly
while planting either small oysters not greater as can be seen in figure 18, which represents
than 2 inches in height, or marketable adults of 5 different shapes found in old shells of C. virginica.
to 6 inches. In the genus Ostrea the difference Very narrow, straight, or slightly curved beaks of
between the two valves is not great, it is greater the kind shown in figure 18-1 are usually formed
in the genus Crassostrea, and extremely pronounced in oysters which grow on soft, muddy bottoms.
in the oyster of uncertain systematic position Extreme development of this type can be seen in
from Australia which Saville-Kent (1893) has the narrow and slender oysters growing under
called "Ostrea mordax var. cornucopiaeformis." 2 overcrowded conditions on reefs (fig. 19). Other
2 I am Indebted to H. B. Stenzel for calling my attention to this species and forms of beaks (fig. 18, 2-4) cannot be associat-
for several suggestions regarding the morphological terminology used In this
chapter. ed with any particular environment. In fully
16 FISHERY BULLETIN: VOLUME 64, CHAPTER II
 o Cent i meters
5

FIGURE I5.-Blue Point oyster (C. virginica) from Great South Bay, Long Island, N.Y. The size of this 5-year-old
oyster is about 10 x 6.6 em. (4 x 3 inches). The shell is strong and rounded; its surface is moderately sculptured.
Left-outside surface of left valve. Right-inner surface of right valve. Small encircled area under the hinge on
the inner surface of right valve is an imprint of Quenstedt's muscle.

grown O. virgmtca the pointed end of the upper

r.v. (flat) valve is always shorter than that of its
opposite member (fig. 17). The angle between
the two beaks determines the greatest extent to
bu. which the valves can open for feeding or respira-
tion and is, therefore, of significance to the oyster.
de.-..........,,-'<-1 f+-#;+-- Ii g. If the oyster shell is oriented in such a way tha t
both of its valves are visible and the beaks point
up and toward the observer, the flat valve with a
I. v. shorter, convex resilifer is the right one and the
cuplike valve with the longer concave resilifer
is the left one. The dorsal margin of the oyster
is the beak or hinge side, the ventra1 fIlargtn tlie
6 opposite. Ii viewed from 'the right (flat) vaive
Centimeters witn ttie hinge 'end pointing" away from the ob-
FIGURE I6.-Cross section below the hinge of an adult C.
server the anterior end of the oyster is at the
virginica. Left valve at bottom, right valve at top of right side of the valves and the posterior is at the
the drawing. The buttress of the right valve fits the left.
depression on the left valve. The two valves are connect- The posterior and anterior parts of the oyster
ed by a ligament (narrow band indicated by vertical shell may also be identified by the position of the
striations) which consists of a central part (resilium)
and two outer portions. Slightly magnified. r.v.-
muscle impressIOn, an oval-shaped and highly
right valve; bu.-buttress; de.-depression or furrow on pigmented area markmg the attachment of the
left valve (I. v.); lig.-ligament. adductor muscle on the inner side of each valve.

MORPHOLOGY AND STRUCTURE OF SHELL 17

 o 5
Centimeters

FIGURE 17.-Side view of a very old and large C. virginica from Stony Creek, Conn. Notice the curvature of the beak,
the depressed resilifer on the lower valve and the protruding resilifer on the upper one. The angle between the beaks
determines the maximum movement of the upper valve. Dimensions: height-25.5 em. (10 inches) and width-6.4
em. (2.5 inches).

The muscle impression is asymmetrically located and G. rhizophorae) that have a wide range of
closer to the posterior end of the valve. This distribution, thrive on various types of bottom,
;;:"'area of theattactlmentOf the adductor muscle and are tolerant to changes in salinity and turbidity
has been called the "muscle s~r." ''Some mala- of water. Certain general relationships between
cologists prefer to use the expression "muscle the shape of the oyster shell and the environment
impression" or "area of attachment" (Stenzel, are, however, apparent in G. virginica. Oysters
personal communication) because the word "scar" growing singly on firm bottom have a tendency
usually means the mark left by healing of an to develop round shells ornamented with radial
injury. The proposed change in terminology ridges and foliated processes (figs. 4, 15). Speci-
does not seem to be desirable because the name mens living on soft, muddy bottoms or those
"muscle scar" has been so well established in which form clusters and reefs are, as a rule,
scientific and popular writings that its abandon- long, slender, and sparsely ornamented (figs.
ment may cause confusion. 19, 21).
The three principal dimensions of bivalves, in- The thickness and strength of the valves of
cluding oysters, are measured in the following G. virginica are highly variable. Shells of oysters
manner (fig. 20): hei ht is the distance between grown under unfavorable conditions are often
the d the ventral valve margin; ~ IS thin and fragile (Galtsoff, Chipman, Engle, and
the maximum distance he anterior and Calderwood, 1947). Likewise, so-called "coon"
posterior margm measured para el WI e oysters from overcrowded reefs in the Carolinas
~ axis; and width is the greatest distance and Georgia are, as a rule, narrow and have light
between the outsides of the closed valves measured shells (fig. 19). Heavy and strong shells are not
at right angles to the place of shell commissure. typical for any particular latitude. They can be
In many popular and trade publications on found on hard, natural bottoms throughout the
shellfish the word "length" is used instead of entire range of distribution of G. virginica. I have
"height", and the word "width" is employed to in my collection shells from Prince Edward Island,
designate the length of the oysters. To avoid
Cape Cod, Delaware Bay, Louisiana, and Texas
confusion the scientific rather than popular
which in shape and strength of valves are in-
terminology is used throughout the text of this
book. distinguishable from one another. Sometimes
The shape of oyster shells and their proportions the growth of shells in length (in anteroposterior
are highly variable and, therefore, are, in some direction) equals or exceeds the growth in height.
cases, of little use for the identification of species. Such specimens, one from Texas and one from the
The variability is particularly great in the species waters of Naushon Island off the Massachusetts
of edible oysters (G. virginica, G. gigas, G. angulata, coast, were found in sticky mud. As can be seen

18 FISH AND WILDLIFE SERVICE

 2

3
4
o Centi meters 3
FIGURE 18.-Four shapes of beaks on left valves of old oysters, C. virginica. I-narrow, short and almost straight;
2-strongly curved to the posterior; 3--of medium width, pointed forward; 4-very broad and slightly curved to the
posterior.

from figure 22, the shells are almost identical in imply the existence of local varieties different
shape and size. in size and shape of shells. There is no evidence,
Oysters are frequently marketed under specific however, to substantiate this claim. So-called
brands or trade names such as Blue Points (fig. "Blue Points" characterized by round shape,
15), Cotuits, Chincoteagues, and others which strong shell, and medium size may be found,

MORPHOLOGY AND STRUCTURE OF SHELL 19

 UMBO
------'1
I
I
1
1
1-1
i§1
iii I
:1:
LENGTH 1
------------ I
I
I
I
---~- --.J

FIGURE 20.-Diagram showing the correct method of

measuring the height, length, and width of oyster shells.
5
for instance, in any part of the coast where
oysters grow singly on hard bottom and are not
crowded. As a matter of fact, in past years
"Blue Points" sold in retail stores actually were
taken from the Chesapeake Bay and North
Carolina. This is also true for "Cotuits" and
other popular brands.
That the shape of oysters cannot be associated
with any particular geographical location is best
shown by the fact that all the kinds represented
in trade, including long and narrow "coon"
oysters which are regarded as being typical for
the tidal /lrreas of the South Atlantic States, are
found in various bays and estuaries of Cape
Cod, Mass. The only shell character that
appears to be associated with the geographical
distribution of the species is the pigmentation
of the interior surfaces of the valves. In North
Atlantic oysters the inner surface is unpigmented
or very lightly pigmented (outside of the place
of attachment of the adductor muscle), while
in South Atlantic and Gulf oysters the dark
brown or reddish pigmentaton of the valves is
more pronounced.
DIMENSIONS
Oysters (0. virginica) of marketable size usually
measure from 10 to 15 em. (4 to 6 inches) in height;
depending on the place of origin an oyster of this
size may be 3, 4, or 5 years old.
As a rule, oysters do not stop growing after
reaching certain proportions but continue to
increase in all directions and, consequently, may
attain considerable size. Such old and very large
oysters are usually found on grounds undisturbed
FIGURE 19.-Several generations of oysters, C. virginica,
growing vertically on muddy bottom of Altamaha by commercial fishing. The largest oyster in my
Sound, Ga. Notice the very long and narrow beak of collection was found in the vicinity of Boothbay
the lowermost shell. Harbor, Maine. Its dimensions were as follows:
20 FISH AND WILDLIFE SERVICE
 Centimeters
FIGURE 21.-Shells of C. giga8 (left) and C. virginica (right) grown on soft, muddy bottom. Note the remarkable simi-
larity in the shape, size, and sculpture of the two species of oysters. The C. giga8 was obtained from the northern
part of Puget Sound and the C. virginica from Georgia. The shells of the two species can be distinguished by the
absence of pigmentation of the muscle impression in C. giga8 and by its lighter shell material.

height-20.6 em. (8.1 inches); height of left and furrows, spines and nodules, or by pigmented
right beak-5.5 em. (2.1 inches) and 4.5 em. spots which repeat themselves with remarkable
(1.75 inches) respectively; length of shell-9.7 regularity. A spiral structure is not restricted to
em. (3.8 inches); maximum width (near the mollusk shells. As a matter of fact, it is very com-
hinge)-6.5 em. (2.6 inches). The total weight mon throughout the animal and plant kingdom as
was 1,230 g., the shell weighing 1,175 g., the meat well as in architecture and art. Examples of a
35.8 g., and the balance of 19.2 g., representing great variety of spirally built organisms and
the weight of sea water retained between the structures are given in the beautifully illustrated
valves. Apparently the largest oyster recorded books entitled "Spirals in nature and art" and
in American literature is the giant specimen from "Curves of life" (Cook 1903, 1914). As the title
the Damariscotta River, Maine, reproduced in of the second book implies, Cook is inclined to
natural size by Ingersoll (1881, pI. 30, p. 32). attach some profound significance to the kind
This shell is 35.5 em. (14.3 inches) in height and of curves found in animal and plant forms. This
11 cm. (about 4.4 inches) in length. view, inherited from the philosophers of the 18th
and 19th centuries, considers the spiral organic
SHAPE OF SHELLS structures as a manifestation of life itself. The
The shells of many gastropods and bivalves are influence of this philosophy persisted among some
spiral structures in which the convolutions of the scientists until the thirties of the present century.
successive whorls follow a definite pattern. The It can be found, for instance, as late as 1930 in the
spiral plan is frequently accentuated by ridges, writings of a French physiologist, Latrigue (1930)

MORPHOLOGY AND STRUCTURE OF SHELL 21

 Centi meters

FIGURE 22.-Two left shells of C. virginica grown on sticky mud. On the left side is the oyster from Karankawa Reef in
Matagorda Bay, Tex.; on the right is the oyster from Hadley Harbor, Naushon Island, near Woods Hole, Mass.
The dimensions of the Texas oyster are 13 by 11.5 cm. (5.1 by 4.5 inches) and for the Hadley Harbor oyster 15.5 by
14.5 cm. (6.1 by 5.7 inches).

who in the book, "Biodynamique generale," at- tangent PG (fig. 23) and radius vector OP is con-
tributes mysterious and not well-defined meaning stant. Another property of this curve which may
to the "stereodynamics of vital vortex." These be of interest to biologists is the fact that distances
speculations contributed nothing to the under- along the curve intercepted by any radius vector
standing of the processes which underlie the for- are proportional to the length of these radii.
mation of shells and other organic structures. D'Arcy Thompson showed that it is possible to
In the earlier days of science the geometric apply the mathematical characteristics of curves
regularity of shells, particularly that of gastropods, to the interpretation of the growth of those shells
had been a favored object for mathematical which follow the pattern of a logarithmic spiral.
studies. Properties of curves represented by the According to his point of view, growth along the
contours of shells, as well as those seen in horns, spiral contour is considered as a force acting at any
in flower petals, in the patterns of distribution of point P (fig. 23) which may be resolved into two
branches of trees, and in similar objects, were components PF and PK acting in directions per-
carefully analyzed. An excellent review of this pendicular to each other. If the rates of growth
chapter of the history of science is given in a well- do not change, the angle the resultant force, i.e.,
known book "On growth and form" (Thompson, the tangent PG, makes with the radius vector re-
1942) in which the reader interested in mathe- mains constant. This is the fundamental property
matics and its application to the analysis of organic of the "equiangular" (logarithmic) spiral. The
forms will find many stimulating ideas. idea forms the basis of Huxley's (1932) hypothesis
Among the array of curves known in mathe- of the interaction of two differential growth ratios
matics, the kind most frequently encountered in in the bivalve shells and also underlies Owen's
the shells of mollusks is the logarithmic or equi- (1953) concept of the role of the growth compo-
angular spiral (fig. 23). The latter name refers to nents determining the shape of the valves.
one of its fundamental characteristics, described Another important characteristic of the growth
by Descartes, namely, that the angle between of bivalves pointed out by Thompson is that

22 FISH AND WILDLIFE SERVICE

 F G general and popular books dealing with bivalve
shells, but the author who introduced it in scien-
tific literature could not be traced. The Greek
word "conchoid", derived from "conch"--shell
and "eidos"-resembling or similar to, implies
the similarity of the curve to the contour of a
molluscan shell.
The curve is symmetric with respect to the 90 0
polar axis (fig. 24). It consists of two branches,
one on each side of the fixed horizontal line CD
to which the branches approach asymptotically
as the curve extends to infinity. The curve,
known as conchoid of Nicomedes, is constructed by
drawing a line through the series of points P and
PI which can be found in the following way: from
the pole 0 draw a line OP which intersects the
fixed line CD at any point Q. Layoff segments
QP=QP1=b along the radius vector OP. Repeat
the process along the radii originating from the pole
o and draw the two branches of the curve by
joining the points. The curve has three distinct
forms depending on whether "a" (a distance OQ
from the pole to the point of intersection of the
polar axis with the fixed line CD) is greater, equal
to or less than b. The formula of the curve if
FIGURE 23.-Logarithmic or equiangular spiral. Expla- b<a, is r=a sec o±b, where r is the locus of the
nation in text. equation and sec 0 is secant of the vectorial angle o.
Sporn (1926) made a detailed mathematical
increase in size is not accompanied by any change analysis of the conchoid curve and considered
in shape of the shell; the proportions of the latter that the curvatures of bivalve shells conform to
remain constant, and the shell increases only in this geometrical type. Lison (1942) rejected this
size (gnomonic growth). This general rule holds conclusion as not supported by observations and
true for many free-moving gastropods and bi- experimental evidence. He quite correctly stated
valves. It is not, however, applicable to sessile that Sporn's work deals exclusively with abstract
forms like oysters, in which the shape of the shell mathematical analyses of curves which in reality
changes somewhat with size, particularly at the are not those found in molluscan shells. If one
early stages of growth, and is greatly modified cuts a bivalve shell at any angle to the plane of
by contact with the substratum upon which the closure of the valves, one obtains the curved
mollusks rest. The plasticity and variability of lines of the two valves (fig. 25) which only remotely
attached forms are probably associated with their resemble the conchoid of Nicomedes and touch
inability to escape the effects of proximate
environment. o
The contour of oyster shell may be either circular
(young C. virginica, O. edulis) or elongated and
irregular. Spiral curvature may be noticed,
however, on a cross section of the lower (concave)
valve cut along its height perpendicular to the
hinge. The curve can be reproduced by covering
the cut surface with ink or paint and stamping
it on paper. The upper valve is either flat or
convex.
The curvature of bivalve shells is sometimes FIGURE 24.-Construction of the conchoid curve of
called conchoid. The term may be found in Nicomedes. Explanation in text.

MORPHOLOGY AND STRUCTURE OF SHELL 23

 is usually confined to one plane parallel to the
plane of opening and closing of the shell.
Among many spirals that can be drawn on the
surface of a shell only one is completely confined
to a single plane. This spiral was called by Lison
the "directive spiral"; its plane is the "directive
plane" of the shell. All other spirals which can
be easily noticed on the shell surface as ridges,
furrows, or as pigmented bands deviate to the
right or left depending on which side of the
directive plane they are located (fig. 26).
By mathematical analysis of the curved surfaces
of various bivalve species Lison arrived at the
FIGURE 25.-Cross section of two valves of Cardium.
general equation 3 of a valve. He observed that
The similarity with the conchoid in figure 24 is super- by itself such an equation may not be helpful to
ficial. biologists unless it can be used for comparing the
shape of the individuals of the same species or in
making comparison between the different species.
each other at the ends. The two branches of Lison stated that in practice it is not necessary to
conchoid (0 and D) join together only in infinity make the involved mathematical computations.
(fig. 24). It is sufficient to compare certain "natural"
Lison pointed out that the shape of the shell characteristics of shells, namely, the directive
may be considered as a whole series (ensemble) plane described above, the plane of closure of
of arches, the curvatures of which are described valves (or commissure plane), and the angle of
by logarithmic spirals of the same parameter which
3 General equation of a valve as given by Lison (1939) is as follows:
have a common origin at the umbo. The latter d=O'oPX; oo-=wo+a; z=zoe plto in which p is a constant and UOt "'0, and Zo
is their common pole. The arches terminate at are the functions which express on cylindrical coordinates the form of the
the edge of the valve. The contour of the valve free edge of the valve when the directive plane is located within the xy and
the origin of the coordinates is at the umbo. (Translation by Paul S.
edge, frequently called the "generating curve", Galtsoff.)

FIGURE 26.-Directive plane of scallop shell, Pecten, viewed from hinge end 2a, and from the broad side 2b. The
arrows indicate the directive plane. (After Lison, 1939.)

24 FISH AND WILDLIFE SERVICE

incidence. The plane of closure of the valves In resume, Lison attempted to prove that the
originates at the umbo and passes between the form of the shell in which the generating curve is
edges of the two opposing valves when they are confined to one plane is determined by three
closed and touching each other. The angle of conditions: (1) the angle of the directive spiral,
incidence, as defined by Lison, is the angle between (2) the angle of incidence, and (3) the outline of
the plane of closure and the directive plane. In the generating curve.
round and symmetrical shells of scallops, pearl Further attention to the problem of the shape
oysters, and other bivalves the directive plane is and formation of the bivalve shell was given by
perpendicular to the plane of closure and the Owen (1953). In general he accepted Lison's
angle of incidence is 90 0 (fig. 26). In the shells conclusions and stated that "the form of the valves
of Cardium orbita, the directive plane forms an should be considered with reference to: (a) the
acute angle of 81 0 and is much smaller in elongated outline of the generative curve, (b) the spiral angle
shells such as Fimbria jimbriata and Trapezium of the normal axis, and (c) the form (i.e., plani-
oblongum. The comparison between the shells spiral or turbinate-spiral) of the normal axis."
can easily be made by recording the contours at The normal axis is considered by Owen with
the free margins of the valves and determining reference to: (1) the umbo, (2) the margin of the
the angle of incidence. mantle edge, and (3) the point at which the great-
To determine the shape of logarithmic spiral of est transverse diameter of the shell intersects the
the valve the shell may be sawed along the direc- surface of the valves. Thus, it can be seen from
tive plane (fig. 27) and the section oriented with this statement that Owen's "normal axis" does
the umbo 0 at lower left. If 8 1 and 8 2 are respec- not coincide with Lison's directive plane except
tive lengths of the two radii the value of para- in bilaterally symmetrical valves (fig. 28). Ac-
meter p can be computed by using the fundamental cording to Owen's view, the direction of growth
equation of logarithmic spiral, at any region of the valve margin is the result of
the combined effect of three different components:
p=log. 8 1 -1og. 8 2 (a) a radial component radiating from the umbo
w and acting in the plane of the generating curve,
(logarithms in this equation are natural, to base e). (b) a transverse component acting at right angles
to the plane of the generating curve, and (c) a
tangential component acting in the plane of the
generating curve and tangentially to it. The
turbinate-spiral form of some bivalve shells is due
to the presence of the tangential component which
in plani-spiral shells may be absent or inconspicu-
ous. Likewise, the transverse component may be
greatly reduced or even absent in the valve.
Thus, from this point of view the great variety of
shell forms may be explained as an interaction of
the three components (fig. 29). Owen's point of

~directive ~ane
no~maI4)(is --.__ .~""'-

atlte~ior I
c ~
FIGURE 27.-Valve of a shell sawed along the directive axis FIGURE 28.-Comparison of directive plane of Lison with
describes a plane logarithmic spiral. According to normal axis (Owen). A-shell not affected by tangen-
Lison (1942). OM-radius vector; T-tangent; tial component; B-shell affected by tangential com-
O-umbo; V-angle between the two radii. ponent.

MORPHOLOGY AND STRUCTURE OF SHELL 25

view is basically similar to Huxley's hypothesis be found by experimental and biochemical studies
(1932) of differential length growth and width which may supply biological meanings to abstract
growth of molluscan shells. Owen correctly mathematical concepts and equations. Experi-
points out an error in Lison's interpretations that mental study of the morphogenesis of shells
the lines of equal potential activities involved in offers splendid opportunities for this type of
the secreting of shell material at the edges of the research.
valves are parallel to each other. This is obvi-
GROWTH RINGS AND GROWTH RADII
ously not the case since all lines of growth of the
lamellibranch shell radiate from a common node Nearly 250 years ago Reaumur (1709) discovered
of minimum growth near the umbo. For this that shells grow by the accretion of material
reason the comparison of bivalves can be more secreted at their edges. Since that time this
conveniently made by using radial coordinates as important observation has been confirmed by
has been shown by Yonge (1952a, 1952b). numerous subsequent investigations. The rings
The mathematical properties of shell surfaces on the outer surfaces of a bivalve shell, frequently
are of interest to the biologist because they may but incorrectly described as "concentric", rep-
provide clues to understanding the quantitative resent the contours of the shell at different ages.
aspects of the processes of shell formation. It can Rings are common to all bivalves but are partic-
be a priori accepted that any organism grows in ularly pronounced on the flattened shells of
an orderly fashion following a definite pattern. scallops, clams, and fresh-water mussels. De-
The origin of this pattern and the nature of the pending on the shape of the shell, the rings are
forces responsible for laying out structural ma- either circular or oval with a common point of
terials in accordance with the predetermined origin at the extreme dorsal side near the umbo
plan are not known. The pattern of shell (figs. 30 and 31). The diagrams clearly show
structure is determined by the activities at the that the rate of growth along the edge of the
edge of the shell-forming organ, the mantle. At shell is not uniform. It is greater along the
the present state of our knowledge it is impossible radius, AD, which corresponds to the directive
to associate various geometrical terms which axis of Lison, and gradually decreases on both
describe the shape of the shell with concrete
physiological processes and to visualize the A
morphogenetic and biochemical mechanisms in-
volved in the formation of definite sculptural and
color patterns. The solution of this problem will

lL

~~~~r-"ormal axis
1
8 8
5

norma.l zone
p
------~y
y ~R
o
FIGURE 29.-Normal axis and the two growth components FIGURE 30.-Diagram of a circular bivalve shell of the
in the shell of scallop. LS-plane perpendicular to the kind represented in Pecten, Anomia, and young C.
plane of the generating curve; N-turning point of the virginica. Radii extending from the umbo to the
llonllave side of the shell shown at right; M and O-aux- periphery of the generating curve are proportional to
iliary radii; P-transverse component; R-radial com- the rate of growth at the edge of a circular shell
ponent; UY-normal axis. From Owen (1953). Radius AD corresponds to the directive axis of Lison.

26 FISH AND WILDLIFE SERVICE

 at various angles along the radii AB, AC, ACI ,
and AB I which end at the periphery of a circle.
Balls placed one in each gutter and simultaneously
released will roll down along the vectors B, B I , C,
Cr, and D. If there is no friction or other form
of resistance, all the balls will reach the periphery
at the same time as the ball dropping vertically
along AD. The acceleration along any of the
vectors, for instance, AB, is found from the
formula t 2 =2/g AD where t IS time and g is
acceleration of gravity.
A similar law, involving a more complex
formula, applies to cases in which the generating
curve is nearly elliptical, for instance, in the
shells of adult oysters. The rate of growth at
different sectors of the periphery of the shell
obviously has nothing to do with the acceleration
of gravity, but the similarity between the length
of the radii which represent the rate of growth
B' along a given direction of the shell and the accelera-
tion along the vectors in the theorem of Galileo
is striking. It appears reasonable to expect that
the Galileo formula may be applicable to the
physiological process taking place near the"edge
of the valve. One may assume, for instance, that
the rate of physiological activities is affected by
the concentration of growth promoting sub-
stances or by enzymes involved in the calcification
FIGURE 31.-Diagram of a shell of adult C. mrgtmca. of the shell and that these factors vary at different
Radii extend from the umbo to the periphery of the points of the mantle edge in conformity with
generating curve. The principal axis AGF shows the Galileo's formula. Experimental exploration of
change in the direction of growth at G. The length of
radii is proportional to the rate of shell growth at the the possibilities suggested by mathematical paral-
edge. lelism may be, therefore, profitable in finding the
solution to the mystery of the formation of shell
patterns.
sides of it along growth radii AC, AB, and ACI ,
AB I . CHANGES IN THE DIRECTION OF
Circular shells in O. virginica may be found only PRINCIPAL AXES OF SHELL
in very young oysters (fig. 32a). Within a few
weeks after setting the shell becomes elliptical, The principal axes of shells of O. virginica are
and as elongation (increase in height) continues not as permanent as they are in clams, scallops,
the principal vector of growth shifts to one side and other bivalves in which the shape of the
(fig. 32b). valves remains fairly constant and is less affected
A series of curves noticeable on round shells by environment than in the oyster. The plasticity
(fig. 32) clearly illustrate the differential rate of of oysters of the species Orassostrea is so great
growth along the periphery of the valve, which that their shape cannot be determined geometri-
increases in size without altering in configuration. cally (Lison, 1949). This inability to maintain
Thompson (1942) found an interesting analogy a definite shape is probably the result of the
between this type of growth, radiating from a sedentary living associated with complete loss
single focal point (the umbo), and the theorem of the power of locomotion.
of Galileo. Imagine that we have a series of In some species of oysters the shells are circular
planes or gutters originating from a single point or nearly circular. In such cases the ratio of
A (fig. 30) and sloping down in a vertical plane the height of the valve to its length is equal to

MORPHOLOGY AND STRUCTURE OF SHELL 27

 a

b
o 0.5 1.0
Centimeters
FIGURE 32.-Two small C. virginica growing attached to tar paper. Maximum dimension of shell: a-D.85 em.;
b-l.D em. At b the principal axis curves to the left.

1.0, as, for instance, in O. rivularis (fig. 8) and In small single oysters less than 10 mm. in
O. (Alectryonia) megodon Hanley (fig. 3) (Olsson, height the principal (normal) axis of growth is
1961). Oysters of the latter species from the clearly marked. All other radii symmetrically
Pacific Coast of Central and South America grow oriented on both sides of the principal axis are
singly, in vertical position, cemented to the rocks indicated by the pigmented bands on the surface
by their left valves. The specimens I collected of the shell. The newly deposited shell, dis-
on Pearl Islands, Gulf of Panama, measured 17 cernible at the periphery of the oyster, forms a
to 18 em. in height and 16 to 17 em. in length. band which is wider at the ventral edge of the
The European flat oyster, O. edulis (fig. 9) usually shell and slightly narrows anteriorly and pos-
forms rounded shells in which the length exceeds teriorly (fig. 32a). With the growth of the oyster
the height. Small, noncommercial species, O. its principal axis is shifted to the side, curves, and
sandwichensis of the Hawaiian Islands and O. is no longer confined to one plane. The curvature
mexicana from the Gulf of Panama, are almost of the valve becomes a turbinate-spiral. Grad-
circular with the tendency to extend in length ually the oyster becomes slightly oval-shaped
rather than in height. Crowded conditions under and asymmetrical.
which these species thrive attached to rocks in a The change in the direction of the principal
narrow tidal zone greatly obscure and distort the axis of growth is not associated with the environ-
shape of their shells. ment since it takes place only in some of the
Small O. virginica growing singly on flat surfaces oysters growing under identical conditions. Oc-
without touching each other are usually round casionally oysters are formed in which the pig-
(fig. 32). In a random sample consisting of 100 mentation along the principal axis is so pronounced
single small oysters (spat about 6 weeks old) that the dark band which marks its position may
varying from 5 to 15 mm. in height and growing be' mistaken for an artifact (fig. 33) while the
on tar paper, the height/length ratio varied from secondary axes are not visible. The shells of
0.6 to 1.2. Nearly half of them (49 percent) were adult O. virginica usually curve slightly to the
perfectly round (HjL ratio=l); in 30 percent the left (if the oyster is placed on its left valve and
ratio was less than 1; and in 21 percent the length viewed from above). Frequently, however, in-
exceeded the height. verted specimens are found in which the growth

28 FISH AND WILDLIFE SERVICE

 The once established principal axis of growth
does not always remain unchanged. Occasionally
old oysters are found in which the direction of
growth had undergone sudden changes of about
90 0 • The change shown in figure 35 took place
when both oysters were about 6 to 7 years old.
The instability of the principal axis of growth
may be even more pronounced. My collection
has an oyster (C. virg~nica) found on the banks of
a lagoon near Galveston, Tex., in which the princi-
pal axis, clearly indicated by pigmented bands on
the surface of the valves, changed its direction at
the end of each growing period. The resulting
zigzag line is clearly visible in the specimen (fig.
36).

DIMENSIONAL RELATIONSHIPS OF
SHELL
Shape of a bivalve shell is often expressed as
a ratio between its height and length or by some
other numerical index. Lison (1942) pointed out
that the shape of an oyster shell cannot be ex-
pressed in precise geometrical terms, presumably
because of its great variability. The "index of
o 5 shape" determined as a ratio of the sum of height
Cent i meters and width of a shell to its length was used by
FIGURE 33.-Principal axis of growth of a C. virginica
Crozier (1914) in studying the shells of a clam,
from Chatham, Mass., is deeply marked by a pigmented Dosinia discus. For the mollusks ranging from 2
band. to 7 cm. in length collected near Beaufort, N. C.
this index varied from 1.24 to 1.28 indicating that
has shifted into the opposite direction (fig. 34). the increase of the species in height and width was
The "normal" oyster (the right side of the figure) directly proportional to the increase in length.
is curved to the left while in the inverted specimen, Such regularity is not found in the shells of adult
shown on the left of the figure, the shell curves C. mrginica taken at random from commercially
to the right. Such "right-handed" oysters are exploited bottoms. For the entire range of
probably common in all oyster populations since distribution of this species in the Atlantic and
they were found in Texas, Chesapeake Bay, Gulf states the index of shape varied from 0.5 to
Narragansett Bay, and Great Bay, N.H. In 1.3. The histogram (columns in figure 37) shows
every other respect the inverted specimens are nearly normal frequency distribution with the
normal and had typically cupped left valves with peak of frequencies at O. 9. No significant dif-
well-developed grooved beaks. There is no evi- ferences were found in the index of shape in the
dence that inversion was caused by mechanical northern and southern populations of oysters
obstruction or some unusual position on the examined separately. The boundary between the
bottom. two groups was arbitrarily drawn at the Virginia-
Complete inversion in bivalves was described North Carolina line. The two curves connecting
by Lamy (1917) for Lucina, Chama, and several the frequency points on figure 37 indicate that in
species of the subgenus Goodallia (fa,m. Astartidae). the southern population the index of shape ex-
It consisted in the appearance of structures, typical tends from 0.5 to 1.3, while in the northern oysters
for the right valve, on the left valve and vice it varies from 0.6 to 1.2. The difference is probably
versa. In the case of C. virginica the structural not very significant, but it may be due to a greater
elements remain unaffected and the inversion is percentage of wild oysters on commercially ex-
limited to the contours of the valves. ploited natural bottoms of the southern states.

MORPHOLOGY AND STRUCTURE OF SHELL 29

733-851 0-64-3
 · :~..~/ .

~ .....

a Centimeters

FIGURE 34.-Left valves of the two large C. virginica from Narragansett Bay, R.I. On the right is a "normal" oyster;
its shell curves to the left. On the left side is an inverted oyster; its shell curves to the right.

Most of the oysters from the North Atlantic and aside for planting will be covered by oysters of
Chesapeake states were taken from bottoms on known size. Since the oystermen usually know
which oysters are regularly planted for cultivation. the number of oysters of various sizes needed to
There are no significant differences in the mean, make up a bushel, the information given below
mode, and median of the two groups (table 1). may be used in determining in advance whether
Contrary to the conditions found by Crozier in the area of the bottom is sufficient to provide space
Dosinia discus, the "index of shape" of C. virginica for their additional growth.
is highly variable. It is self-evident that the area of the valve in-
creases proportionally to the increase in its linear
SHELL AREA
dimensions. For determining the area a piece of
I nformation regarding the approximate area thin paper was pressed against the inner surface
of an oyster shell of known height may be useful of the right (flat) valve and the outlines were
to oyster growers who want to determine in ad- drawn with pencil The area wRS mefiSiired with
vance what percentage of the bottom area set II; planimeter. The<:'lutljnes of small shells were
~laced over ;~h paper and ~e number of milli-_
TABLE I.-Index of shape (height+width) of oysters taken by ~r squares coJ!!!..ted..:..-
length
commercial fishery . The relationship between the height and shell
area (fig. 38) is represented by an exponential
Locality Mean Standard Mode Median curve of a general type y= axb which fits many
deviation
.--------- --- empirical data. The y in the formula is the shell
Northern grounds...__ ........... 0.87 0.05 0.94 0.09 area, and the x is the height. The parabolic
Southern grounds________________ 0.87 0.02 0.94 0.9
nature of the curve is demonstrated by the fact

30 FISH AND WILDLIFE SERVICE

 o 5
Cent i meters

FIGURE 35.-Two upper (right) shells of old C. virginica from Chesapeake Bay (left) and Matagorda Bay, Tex. (right).
The direction of growth changed suddenly about 50° to the left in the Chesapeake oyster and about 75° to the right in
the Texas oyster.

that the log/log plot (fig. 39r fits a straight line. ships can be adequately expressed by the formula
The numerical values of factors a and b were of heterogenic growth, y= bxk • According to No-
found to be equal to 1.25 and 1.56 respectively. mura's (1926a) interpretation of the growth of the
The formula reads, there;ore, y= 1.25x1. 56 • As clam Meretrix meretrix, the constant b in this
a convenience to the reader who may be interested formula represents the effect of the environment
in finding directly from the curve the average area while k is a factor of differential growth. No-
occupied by a shell of a given height, the data mura's conclusions may be applicable to other bi-
computed from the equation can be read from the valves, and if confirmed by further studies this
curve in figure 38. The measurements are given method may become useful for quantitative de-
both in centimeters and inches. The data refer
to the random collection of live oysters from the TABLE '2.-Height and shell area oj northern oyster.5 com-
puted by usin(/ the equation y = 1.25x I..l6
coastal areas between Prince Edward Island,
Canada, and the eastern end of Long Island Height Area
Sound (table 2).
Crn. Inches Cm. 2 In.'
The relationship between the height and area 5 . . . 1.97 15.4 2.39
6 • . _ 2.36 20.5 3.18
of the upper valve of G. virginica is in agreement 8....• _ 3.15 32.3 5.01
10 _ 3.94 45.4 7.04
with the findings of other investigators (New- 12 _ 4.72 00.3 9.35
14. . . _ 5.51 76.7 11.9
combe, 1950; Nomura, 1926a, 1926b, 1928) who 16 _ 6.30 94.5 14.6
18 . _ 7.09 113.5 17.6
concluded that in several marine and fresh-water 20 _ 7.87 133.8 20.7
bivalves and gastropods the dimensional relation-

MORPHOLOGY AND RTRUCTURF. OF SHELL 31

 50 ~- -0 N. ATLANTIC

------ S. ATLANTIC
Ul
a
GULF
...J 40
...J
W
:I:
Ul
u.
0 30
a::
w
CD
:!:
::::> 20
z

10

0 0.5 . 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

INDEX OF SHAPE OF C. virginica

FIGURE 37.-Histogram of the distribution of the index of

shape (height + width) of shells of C. virginica from the
length
o 5 Atlantic Coast. Frequency distribution of the index of
Cent imeters North Atlantic oysters (open circles) and South Atlantic
oysters (points) are shown by two separate curves.
FIGURE 36.-Shell of an adult C. virginica showing periodic
changes in the direction of the principal axis of growth.
Note the zigzag line of pigmented bands in the middle of of this explanation was presented by the authors
the valve. Actual dimensions: 8.5 by 6 em. (3.25 by 2.5 or by Ranson (1939-41), who fully accepted the
inches). theory without making additional studies and
stated positively that chalky deposits are formed
terminations of the effect of local conditions on wherever there is a local detachment of the mantle
growth and shape of shells. from the valve.
Considering the possibility that the mantle may
CHALKY DEPOSITS
be more easily detached from the valve if the
The glossy, porcelainlike inner surface of an oyster is placed with its lower (cuplike) side upper-
oyster shell is frequently marred by irregularly most, Korringa (1951) made a simple field ex-
shaped white spots which consist of soft and periment. Tn one tray he placed 25 medium
porous material of different appearance and text- sized O. edulis in their normal position, with their
ture than the surrounding shell substance. These cupped valves undermost; the other tray contained
areas are called "chalky deposits". They are an equal number of oysters resting on their flat
very common in C. virginica and O. edulis. Since valves. At the end of the growth season he ob-
the first record of their presence in edible oysters served no significant differences in the deposition
made by Gray (1833) they have been mentioned of shell material in the oysters of the two groups.
frequently by many biologists. Recent review To determine whether chalky deposits are
of the literature on the subject is given by Kor- formed in places of partial detachment of the
ringa (1951). mantle, I performed the following experiment:
The exact location of chalky deposits is of Small pieces of thin plastic about 1 cm. 2 were bent
interest since some speculations regarding their as shallow cups and introduced between the
role and orIgin are based on the position they occu- mantle and the shell of C. virginica. Tn 10
py on the shell. Orton and Amirthalingam oysters the cups were inserted with the concave
(1927) assumed that chalky material is formed in side facing the mantle, in another 10 oysters the
the places where the mantle loses contact with position of the cup was reversed, Le., the concave
the shell. No experimental evidence in support side faced the valve. The oysters were kept for

32 FISH AND WILDLIFE SERVICE

 HEIGHT IN 55 days in running sea water in the laboratory.
2 3 4
During this time they fed actively and had con-
siderable shell growth along the margin of the
120
.. 18
valves. After their removal from the shells the
cups were found to be covered with hard calcite
16
100 .. deposits on the sides facing the mantles. No
chalky material was found on cups or on the sur-
14
face of valves adjacent to the area of insertion.
1201-:
On the other hand, conspicuous chalky areas
~
were formed along the edge of the shell in places
L5 10
<l
l:! where the opposing valves were in close contact
II:
<l 60
<l
with each other (fig. 40). It is clear from these
...J ...J
...J
.... 8
...J
.... observations that the detachment of the mantle
:J: :J:
en
.... en from the inner surface of the shell does not result
40
: . ... 6 in the deposition of chalky material and that such
deposits may be laid in the narrowest space of
4 shell cavity where the two valves touch each other.
20
.. .. 2
Suggestions that chalky deposits result from
secondary solution of calcium salts of the shell
(Pelseneer, 1920) or that their formation is
01;----+2-74-~6"---+8-+'10,--I1\;-2-fA
I4-T.16;--'11>8---'2~0:--'0 somehow related to the abundance of calcareous
HEIGHT IN CENTIMETERS
material in the substratum (Ranson, 1939--41,
FIGURE 38.-Shell area in cm. 2 plotted against height of
shells in em. Inch scales are on top and on the right.

0 0
00

100 . 0 00

90 0°8 0
0
80 00
0
7
6

NE 50
~
e:t 40
l.LJ
Q:
<l:
30

20
d.d.

10
o 5
Cent i meters

FIGURE 40.-Chalky deposits (ch. d.) on the newly

FIGURE 39.-Logarithmic plot of shell area against shell formed shell at the edge of the valve, and near the
height. muscle attachment.

MORPHOLOGY AND STRUCTURE OF SHELL 33

1943) are not supported by evidence. The inner assumptions with reference to C. virg~mca was
surface of bivalve shells may become slightly tested by studing the relative frequency of the
eroded due to the increased acidity of shell liquor occurrence of chalky deposits on the left and
when the mollusk remains closed for a long time, right valves and by estimating the extent of
but the erosion is, however, not localized; it these deposits in different parts of the valves.
occurs over the entire shell surface. As to the The collection of shells studied for this purpose
effect of the abundance of lime in the substratum comprised several hundred adult specimens from
on the formation of chalky deposits, one must various oyster beds along the Atlantic and Gulf
remember that the concentration of calcium salts coasts. For determining the distribution of
dissolved in sea water is fairly uniform and that chalky areas the inner surface of the valves was
calcium used for building of shells is taken arbitrarily divided into four quadrants shown in
directly from the solution (see p. 103). Under figure 42 and designated as follows: A-dorso-
these conditions the abundance of calcium car- posterior; B-dorsoanterior; C-ventroposterior;
bonates in bottom deposits cannot have any and D-ventroanterior. The following five classes
effect on the formation of shell. corresponding to the degree of the development
Chalky areas of shell do not remain unchanged. of chalky deposits in each quadrant were
They become covered by hard substance and in established:
this way they are incorporated in the thickness
No deposits within the quadrant____________ 0
of the valves (fig. 41). 1 to 25 percent of the area covered with
Korringa's theory (1951) that the oyster deposits_ _______________________________ 1
deposits chalky material ". . . when growing 26 to 50 percent of the area covered with
older, in its efforts to maintain its efficiency in deposits_ _______________________________ 2
functioning" and that " . . . where possible the 51 to 75 percent of the area covered with
deposits_ ______________ _________________ 3
oyster always uses soft porous deposits when 76 to 100 percent of the area covered with
quite a lot of shell volume has to be produced . . ." depo~ts________________________________ 4
is based on the assumptions: (1) that chalky
deposits most frequently develop in the area With a little practice it was easy to select the
posterior to the muscle attachment, (2) that the correct class by visual examination. The first
layers of chalky material are more numerous in question was whether there is any difference in
cupped than in flat oysters, (3) that in the area the frequency of occurrence and extent of chalky
of the exhalant chamber (in the posteroventral deposits on right and left valves. For this
quadrant of the shell) the oyster attempts to purpose the entire surface of the valve was exam-
decrease the distance between the two valves by ined and classified. Chalky deposits were
rapid deposition of shell material, and (4) that found as often on the right as on the left valve
chalky material is used by the oyster "as a measure of C. virginica. This is shown in table 3 which
of economy, as a cheap padding in smoothing out summarizes the observations made on 472 shells
the shell's interior." The validity of these collected at random at oyster bottoms along the

a Cent imeters
5

FIGURE 41.-Left valve of an old C. virginica cut along the principal axis of growth. Chalky areas on both sides of the
hypostracum (dark platform for the attachment of the adductor muscle) are enclosed in the thin layers of hard
crystalline material. Hinge on the right. Natural size.

34 FISH AND WILDLIFE SERVICE

 · '. than in flat ones and can be found principally in
the area in front of the cloaca, quadrant C accord-
ing to our terminology. No such differences in
the place of formation or in the type of shell
could be observed in G. virginica.
From the observations on oysters of Prince
Edward Island, Medcof (1944) concluded that
B ':'.
chalky deposits are normal parts of shells and
that they have "functional importance" in pre-
serving "a size relationship between meats and
shell cavity" and in regulating "the curvature of
the inner face of the shell throughout the oyster's
life." There could be no argument about the
first conclusion that chalky deposits are normal

c parts of the oyster shell. The fact that they

appear during the first weeks of the oyster's life
confirms this statement. The second conclusion
that they preserve the curvature of the shell is
FIGURE 42.-Four arbitrary quadrants of the inner surface impossible to prove without careful study of a
of shell used for estimating the distribution and extent large number of shells. In comparing the con-
of chalky deposits. tours of the shells of New England and Chesapeake
Bay oysters with and without chalky deposits,
Atlantic Coast from Long Island Sound to Georgia. I failed to notice any significant difference between
Nearly one-half of the total number of valves the two groups.
examined (48 percent of left and 53 percent of Japanese investigators (Tanaka, 1937, 1943)
right valves) were free of the deposits. (The found great variability in the distribution of
percentage of oysters without chalky deposits chalky deposits in G. giga8 and G. futamien8is.
was not determined because in many shells of the Large porous areas may be found in the shells
collection the valves had separated and could of these species near the anus, in front of the
not be arranged in pairs.) In about 25 percent labial palps, or near the gonads. There seems
of the total number of shells the chalky deposits to be no evidence that they occur primarily in
cover less than one-quarter of the valve area. one particular place of the valve. These obser-
Larger deposits occurred in diminishing number vations agree with my observations on G. virginica.
of shells; those covering more than three-quarters
CHAMBERING AND BLISTERS
of avaihi,ble space (class 4) comprised less than
3 percent of the total number examined. The French word "chambrage" or chambering
There was no particular area on the valve has been used by European biologists to describe
surface where chalky deposits were formed more shallow cavities, mostly in the cupped valves of
often than in any other place. The differences O. eduli8. The cavities are usually filled with sea
in the frequency of their occurrence in different water and putrified organic material. In the
quadrants of a valve were not significant. museum specimens these spaces are dry and filled
In O. edulis, according to Korringa, chalky with air. Sometimes only one chamber is found,
deposits form more often in deep (cupped) shells but occasionally an entire series of cavities may
be present. The chambers may be invaded by
TABLE 3.-Percent of valves of C. virginica with chalky
deposits tube-forming annelids living in the oyster (Houl-
bert and Galaine, 1916a, 1916b). The successive
Area of valve covered by chalky deposits
layers of shell material in the chamber are not in
Item
Class 1 Class 2 Class 3 Class 4 contact with each other but surround an empty
(1-25 (26-50 (51-75 (76-100
percent) percent) percent) percent) space. This gives the impression that the body
--------- of the oyster had shrunk or retracted and occupies
Left valve.. __ . __ . ______ . ________ 25.9 13.6 9.8 2.8
Right valve.___________________ ._ 24.9 12.1 8.4 1.5 only a small portion of shell space. This view is
generally accepted by European oyster biologists

MORPHOLOGY AND STRUCTURE OF SHELL 35

 (Korringa, 1951; Orton, 1937; Orton and Amirtha- relatively small amounts of building material.
lingam, 1927; Worsnop and Orton, 1923), who What advantage O. iridescens obtains from this
agree that chambering is caused by the shrinkage type of structure is of course a matter of specu-
of the body, withdrawal of shell-forming organ, lation.
and deposition of partitions. Salinity changes Chambers found in O. virginica consist of
were suggested by Orton as one of the principal irregular cavities containing mud or sea water.
causes of chambering, and shrinkage due to Such formations are called blisters. Blisters can
spawning was also considered by Korringa as a be artificially induced by inserting a foreign
probable factor. These conditions have not been object between the mantle and the shell (see p. 105).
reported for O. virginica. I did not find any They are also caused by the invasion of shell
evidence that chambers or blisters in the American cavity by Polydora (see p. 422) or by perforations
oyster are associated with shrinkage or other of the shell by boring sponges and clams (p. 420).
body changes.
STRUCTURE 0 F SHELL
It is interesting to add that some taxonomists
of the middle of the past century (Gray, U~33; For more than a hundred years the structure of
Laurent, 1839a, 1839b) were so puzzled by the the molluscan shell was an object of research by
presence of chambers that they compared cham- zoologists, mineralogists, and geologists. Several
bered oyster with Nautilus and even suggested reviews of the voluminous literature (Biedermann,
the possibility of some family relation between the 1902a, 1902b; B~ggild, 1930; Cayeux, 1916; Haas,
latter genus and Ostrea! 1935; Korringa, 1951; Schenck, 1934; Schloss-
An interesting shell structl\I'e consisting of a berger, 1856) deal with the problem from different
series of chambers near the hinge end is found in points of view. Recently these studies have been
the Panamanian oyster, O. iridescens. The loca- extended by the use of X-ray and electron micro-
tion of chambers and the regularity at which they scope. The methods, especially those of electron
are formed as the shell grows in height can be seen microscopy, opened entirely new approaches par-
in figure 43 representing a longitudinal section of ticularly with reference to the structure of the
the valve made at a right angle to the hinge. organic constituents of the shell (Gregoire, 1957;
This type of chambering is obviously a part of a Gregoire, Duchateau, and Florkin, 1950, 1955;
structural plan of the shell and is not a result of an Watabe, 1954).
accidental withdrawal of the oyster body or of an Terminology of molluscan shells is somewhat
invasion by commensals. Arch-forming septae confusing depending whether the emphasis is
of the chambers apparently contribute to the placed on morphological, crystallographical, or
strength of the hinge and at the same time require mineralogical properties. The names of different

o Centimeters
5
FIGURE 43.-Shell of O. iridescens cut at right angle to the hinge. Note a series of empty chambers at the hinge area.
Specimen from the Gulf of Panama.

36 FISH AND WILDLIFE SERVICE

layers of shell described in this chapter are those layer the prisms are wedge-shaped and slightly
which are found in more recent biological publi- curved (fig. 46). Conchiolin adhering to the
cations (Korringa, 1951; Leenhardt, 1926). prisms can be destroyed by boiling in potassium
The shell of the oyster consists of four distinct hydroxide solution and the prisms separated
layers: periostracum, prismatic layer, calcite- (Schmidt, 1931). Their shape and size are very
ostracum, and hypostracum. The periostracum variable.
is a film of organic material (scleroprotein called The optical axes of the prism are, in general,
conchiolin), secreted by the cells located near the perpendicular to the plane of the prismatic layer,
very edge of the mantle. The periostracum is but in places they are irregularly inclined toward
very poorly developed in O. virginica and cannot it.
be found in old shells. It covers the prismatic Calcite-ostracum, called also a subnacreous
layer which can be best studied by removing from layer (Carpenter, 1844, 1847), makes up the major
the edge of an oyster a small piece of newly formed part of the shell. The layer consists primarily of
shell. Microscopic examination reveals that the foliated sheets of calcite laid between thin mem-
prismatic layer is made of single units shown in branes of conchiolin. The separate layers are
figure 44. Each prism consists of an aggregate of irregularly shaped with their optical axes in ac-
calcite crystals (Schmidt, 1931) laid in a matrix cidental position (B~ggild, 1930). In a polished,
of conchiolin which after the dissolution of mineral transverse section of the shell of O. virginica the
constituents in weak hydrocWoric acid retains the folia are laid at various angles to the surface
general configuration of the prisms (fig. 45). The (fig. 47). This layer is frequently interrupted by
double refraction of the walls of empty prisms is soft and porous chalky deposits (upper two layers
pronounced and causes slight iridescence notice- of fig. 47) which appear to consist of amorphous
able under the microscope. In a well-formed material. It can be shown, however, that chalky
deposit is formed by minute crystals of calcite
oriented at an angle to the foliated lamellae of the
hard material.
Hypostracum is a layer of shell material under
the place of the attachment of the adductor muscle.
In the shells of O. virginica the layer is pigmented
and consists of aragonite (orthorhombic calcium
carbonate, CaCOs).
For many years oyster shells were considered to
be composed entirely of calcite (B~ggild). Re-
cently Stenzel (1963) has discovered that on each
valve of an adult O. virginica aragonite is present
as padding of the muscle scar, in the imprint of
Quenstedt's muscle, and in the ligament.
As the oyster grows the adductor muscle in-
creases in size and shifts in the ventral direction.
The new areas of attachment become covered
with aragonite while the older, abandoned parts
are overlaid with the calcite. The progress of
the muscle from hinge toward the ventral side can
be clearly seen on a longitudinal section of the
shell where it can be easily distinguished by its
darker color and greater hardness of the secreted
material (fig. 48).

ORGANIC MATERIAL OF THE SHELL

o Millimeters
0.5
After the removal of mineral salts of the shell by
FIGURE 44.-Prismatic layer at earlier stages of calcifica- weak acids or by chelating agents, such as sodium
tion. C. virginica. versenate, the insoluble residue appears in the

MORPHOLOGY AND STRUCTURE OF SHELL 37

 , I 1

'II' t ers 03
M lime

FIGURE 45.-Photomicrograph of a thin picce of prismatic layer after the dissolution of calcium carbonate in weak acid,
C. virginica. The walls retain the shape of the prisms and are iridescent.

38 FISH AND WILDLIFE SERVICE

 test for chitin (intense violet coloration after
treatment for 24 hours in diaphanol [chlorodioxy-
acetic acid], followed by a solution of zinc chloride
and iodine), does not confirm these findings (Lison,
1953).4
To the naked eye and under the light micro-
scope the conchiolin appears as amorphous, viscous
and transparent material which hardens shortly
after being deposited. Using the electron micro-
scope technique, Gregiore, Duchateau, and Florkin
(1955) found that the conchiolin of gastropods and
bivalves consists of a fine network with many
meshes of irregular shape and variable dimensions.
This is, however, not the case in oyster shells.
Conchiolin of the genus Ostrea lacks meshes and
under the electron microscope is of uniform ap-
pearance (personal communication by Gregoire).
Cross sections of decalcified shells of C. virginica
show a distinct difference between the staining
properties of the conchiolin of the prismatic and
0.5 calcite-ostracum layers. On the cross sections of
Millimeters shell shown in figure 49 the two parts can be
FIGURE 46.-Cross section of a piece of young shell of C. recognized by the typical foliated appearance of
virginica (mounted in bakelite and ground on a glass the calcite-ostracum and the meshlike structure
wheel with carborundum, about 80 x). Periostracum of the prismatic layer. In the preparation stained
(top line), prismatic layer (middle), and calcite-ostracum with Mallory triple dye the organic matter of
(lower).
the walls of the prisms are stained reddish-brown
while the foliae of the calcite-ostracum are bluish.
form of thin, homogenous sheets of organic material
Differential staining indicates the difference in
kept together like pages of a book. This sub-
the chemical composition of the two parts.
stance, discovered in 1855 by Fremy, is known as
The amount of conchiolin in the oyster shell was
conchiolin. The name is applied to the organic
studied by several investigators. As early as
material insoluble in water, alcohol, ether, cold
1817 Brandes and Bucholz estimated that organic
alkaline hydroxides, and dilute acids. In the
material of the shell constitutes about 0.5 percent
literature it appears also under the names of
of the total weight. Schlossberger (1856) found
conchin, periostracum, epidermis, and epicuticula.
6.3 percent of organic matter in the prismatic
Conchiolin is a scleroprotein, the structural for-
layer of the oyster but only from 0.8 to 2.2 percent
mula of which has not yet been determined. The
elementary analysis of conchiolin of O. edulis in the calcite-ostracum. According to Douville
(Schlossberger, 1856) is as follows: H, 6.5 percent; (1936), the albuminoid content of the oyster shell
C, 50.7 percent; N, 16.7 percent. Wetzel (1900) is 4.8 percent.
found that conchiolin contains 0.75 percent of According to the determinations made by A.
sulfur and Halliburton (quoted from Haas, 1935) Grijns for Korringa (1951), the conchiolin content
assigned to it the following formula: C30 , H 48 , N g , of the prismatic layer of O. edulis varied from 3.4
au, which also appears in the third edition of to 4.5 percent against the 0.5 to 0.6 percent in the
"Hackh's chemical dictionary" (Hackh, 1944). calcite-ostracum. The conchiolin content was
Similarity of conchiolin to chitin' leads many calculated from the percentage of N (by Kjeldahl
investigators to an error in ascribing chitinous method) multiplied by 6.9. The results of my
composition to structures which were found in-
determinations of the weight of organic material
soluble in alkaline hydroxides and dilute acids.
Thus, the presence of chitin was reported in the • Inasmuch as the same reaction is obtained with cellulose and tunicine,
additional tests should be made using Lugol solution and 1 to 2 per cent
shell and ligament of Anodonta, Mya, and Pecten sulphuric acid (H2S0.). With this test chitin Is colored brown, while cellu-
(Wester, 1910). The application of the Schulze's lose and tunlclne are blue.

MORPHOLOGY AND STRUCTURE OF SHELL 39

 I I I I
o .. 0.3
Millimeters
FIGURE 47.-Cross section of the shell of adult C. virginica em.bedded in bakelite and polished on a glass wheel with carbo-
rundum.. Two upper layers consist of chalky deposits.

after decalcification of the calcite-ostracum of the new growth of shell which has not yet com-
C. virginwa shells from Long Island Sound and pletely calcified.
Cape Cod waters are in agreement with those The role played by conchiolin in the deposition
given for O. edulis. The content of conchiolin in of calcium salts in the form of calcite or aragonite
my samples varied from 0.3 to 1.1 percent with presents a very interesting problem which has not
the mode at 0.6 percent. For these analyses 23 yet been solved. Recent electron microscope
pieces of shell were taken from 16 adult oysters studies of pearl oyster shells made by Gregoire
not damaged by boring sponge. The samples show that the organic material in which aragonite
varied in weight from 0.5 to 15 g. crystals are laid (Gregoire, Duchliteau, and
Higher percentage of conchiolin in the prismatic Florkin, 1950) is arranged as a series of bricklike
layer may be expected because this layer represents structures. No such arrangement has been de-

40 FISH AND WILDLIFE SERVICE

 o Centimeters
5

FIGURE 48.-Left valve of O. (Alectryonia) megodon cut along the principal axis of growth. Hypostracum (dark striated
layer) forms a pronounced platform for the attachment of the adductor muscle, and can be traced to its original position
in the young oyster (right). Chalky deposits are regularly arranged between the layers of calcite. Also see fig. 41.

scribed for calcite shells. Present knowledge of Taking advantage of the fact that both calcite
the chemistry of the organic constituents of the and aragonite are present in the two distinct
shell is inadequate. It seems reasonable to layers of shell of the fan oyster (Pinna) and of the
assume that conchiolin like other proteins is not pearl oyster (Pinctada) , the French investigators
a single chemical substance common to a large (Roche, Ranson, and Eysseric-Lafon, 1951) at-
number of organisms, but that it differs specifi- tempted to determine whether there is a difference
cally from animal to animal and may even vary in the chemical composition of the organic material
in the different parts of the same shell. of the two layers of the shell of the same species.
The analysis of amino acids obtained by hy- They found that tyrosine and glycine occur in
drolysis of conchiolin prepared from decalcified higher concentrations in the prismatic layer than
shells showed (Roche, Ranson, and Eysseric- in the nacreous part of shells. In the prismatic
Lafon, 1951) that there is a difference in the shells layer of calcite portion the content of tyrosine
of the two species of European oysters, O. edulis varies between 11.6 and 17.0 percent and that of
and C. angulata (table 4). glycine between 25 and 36 percent. In the
nacreous part made of aragonite the concentration
TABLE 4.-Amino acids from the conchiolin of two species
of oysters of tyrosine was from 2.8 to 6.0 percent and that
[In parts 01100 parts of protein according to Roche, Ranson, and Eysseric- of glycine varied between 14.9 and 20.8 percent.
Lafon (1951)]
The significant differences in the contents of the
Amino acids Crassos/rea Os/rea two amino acids in the two parts of the shell
angula/a edulis
----------------1----- ---- may provide a clue for further studies of the role
Arginine_
Histidine_________ __ __ __ __ __ __ ____ __ __ __ __ __
0.45
___ __ __
2_ 90
0.65
of the organic component on the mineral form in
Lysine____________________________________________
Glycine___________________________________________
3.55
15.70
4.30
15.70
which the calcium carbonate is deposited by the
Leucine___________________________________________
Tryptophane______________________________________
0.51
0.48
_ mantle.
Tyrosine__________________________________________ 3.27 3_ 05 MUSCLE ATTACHMENT
Valine_____________________________________________ 0.95 _
Cystine_ 0.98
Methioninc_ 1.77 1.62 The place of attachment of the adductor muscle
or muscle scar is the most conspicuous area of the

MORPHOLOGY AND STRUCTURE OF SHELL 41

 figure were obtained in the following manner:
the periphery of the impression was circumscribed
with soft pencil; a piece of transparent Scotch
adhesive tape was pressed on the impression and
the outline was lifted and mounted on cross-
section paper; the area occupied by the impression
was measured by counting the number of squares.
Using this method, I obtained the replicas of
muscle impressions from 169 shells taken at
random from various oyster beds of the Atlantic
and Gulf Coasts. The impressions are arbitrarily
arranged in four series (A-D) according to their
shape and size. The impression areas of round
and broad shells are shown in the two upper rows,
A and B; those of long and narrow shells are
arranged in the two lower rows, C and D.
It may be expected that the larger is the shell
the greater is the area of muscle impression.
The relationship, as can be seen in fig. 51, is
rectilinear although the scatter of plotted data
is considerable and the variability increases with
the increase in size. The ratio of muscle impres-
.:.:; .. .: =.. :
.: : .. : ,= .. :
:.: :
~. :
sion area to shell surface area varies from 8 to 32
. '.:
:. : :
'
. .. . " ; ~',,: ,:.; -. . . '. with the peRk of frequency distribution at 16 to

,tlt\:l~D;;::· · · · \.·;.~. ;::S.::·i:·;-\:,,,7 18 (fig. 52).

A small oval and unpigmented area on the

o Millimeters
1.0

FIGURE 49.-Cross section of shell of an adult C. virginica

after decalcification in weak acid, Mallory triple stain.
Conchiolin of the prismatic layer is reddish-brown; that
of calcite-ostracum is bluish. o C7
oyster shell. In C. virginica, C. angulata, and
many other species this area is highly pigmented;
in O. edulis, C. gigas, pigmentation is either
absent or very ligh~.
The muscle scar in C. virginica is located in the
c
o oo
posteroventral quadrant of the shell (figs. 15, 21,
33). To a certain extent the shape of the scar
reflects the shape of the sh~il, being almost round
o o
in broad and round oysters and elongated in
narrow and long shells. The area of scar is b
slightly concave on the side facing the hinge and Centimeters

convex on the opposite, i.e., ventral side. Curved FIGURE 50.-Variations in shape and size of muscle scars
growth line, parallel to the curvature of the on the shells of C. virginica. Rows A and B show the
ventral edge of the valve, can be seen on the types of scars normally found on broad and rounded
surface. They are most pronounced in the ventral shells, the length of which is almost equal to or exceeds
the height. Rows C and D are the scars often found
part of the muscle impression. Size and shape of on long and narrow shells in which the height exceeds
the scar is variable and often irregular (fig. 50). the length. Replicas of scars were made from shells
The outlines of the impressions shown in this collected at random.

42 FISH AND WILDLIFE SERVICE

 TABLE 5.-Chemical composition oj oyster shells in percent
5.01- oj shell weight
[From Hunter and Harrison, 1928]

g 0
Constltuen ts Sampie 1 Sample 2
4.0
o ..
,
"l!<.> o
00 0
0 0
0
0
0 0
0.045
38.78
0.043
38.81
otI 0 0
0
0° 0 ottP 00
-- 0.0025
l3' 3.C 0o§o <) 0, 800 0 0 ° 00
-----ii~ii--­
0.09
ll: ,0 , 0.183 0.189
~
'...." 00

o
00
0° 0

0
0 0000
000
000 0
0
0.009
0.075
0.009
0.073
""
~
o 0 0 0 0 000
0
0.570 0.580
0.0009
~
2.0 o 0 og 00°00'; a 0.0034 0.0035
000 0 ~o~o O! 357.19
o :~{o 0 % ~ooo ~~
0000 000 00 ~ 00 0.196
eo 0 °00'6 0 0 0
1.0 0:eoo og 0 1. 41 1. 51
OOOQ:lo 08:' 0.27 0.28
00 0

1 Loss above 110° C. Ignited.

00 2 Loss to 100° C.
10.0 20.0 30.0 40.0 50.0 60.0 3 Average for samples 1 and 2.
SHELL AREA, cm.2

FIGURE 5l.-The relationship between the area of muscle CHEMICAL COMPOSITION

scar and the area of the shell of C. virginica. The oyster shell consists primarily of calcium
carbonate, which composes more than 95 percent
dorsal half of each valve is the imprint of a of the total weight of the shell. The balance is
vestigial muscle in the mantle, discovered in 1867 made up by magnesium carbonate, calcium sul-
by Quenstedt in the valves of the (larly Jurassic fate, silica, salts of manganese, iron, aluminum,
oyster, Gryph,aea arc11:ata Lamark, and found by traces of heavy metals, and organic matter.
Stenzel (1963) in C. virginica. In my collection Several analyses of oyster shell found in the
of living C. virginica the imprint is hardly visible literature are incomplete, particularly with refer-
(figs. 15, 21, and 22). Slight adhesion of the ence to trace elements. Analysis made for the
mantle to the valve indicates the location of this U.S. Bureau of Fisheries by the Bureau of
area which Stenzel calls "imprint of Quenstedt's Chemistry of the Department of Agriculture and
muscle." published in 1928 (Hunter and Harrison, 1928) is
given in table 5.
Dead oyster shells buried in the mud of the
inshore waters of Texas and Louisiana are exten-
32
~~ sively dredged by commercial concerns primarily
30
for the manufacture of chicken feed. Analysis of
28
I-- these shells as they are received at the plant after
26
thorough washing in sea ~ater is given in table 6.
24
- The calcium carbonate content of these shells is
~ 22
-J
~
probably lower than in live oysters due to their
~ 20
erosion and dissolution of lime in sea water.
~ 18
The chloride content IS affected by the retention of
~ 16
a 14 TABLE 6.-Chemical composition oj mud shell.~ received at
~ 12 f-- the plant oj Columbia-Southern Corporation at Corpus
10
Christi, Tex.
[Percent of constituents in samples dried at 110° C.]
8
1---
6 Chemical Percent
-
4
93.88
2

o 2 4 6
d 8 10 12 14 16 18 20 22 24 26
~
28 30 32
0.48
0.88
1.40
0.32
SHELL AREA ':"MUSCLE ATTACHMENT AREA 0.27
0.46
1. 60
FIGURE 52.-Frequency distri):mtion of the ratio of muscle
scar area vs. shell area in the shells of C. virginica of
(Analysis supplied by Columbia-Southern Corporation and copied with
Atlantic and Gulf States. their permission.)

MORPHOLOGY AND STRUCTURE OF SHELL 43

 t~ese salts in the shells after thorough washing TABLE 8.-Chemical composition of shells of O. edulis (in
percent of ash residue)
wIth sea water of greatly variable salinity. The
percent of silica, aluminum, and iron, which are Constituent Sample Sample Sample Sample
1 2 3 4
also higher than in the analyses of shells of live -----------
oysters, is at least in part influenced by the gaco.___________________________ 98.60 97.65 96.54 97 00
Ma.i!oO.lo-------.---------------- 1. 21 .
efficiency of plant operations in removing mud g •• -------- -- ----ii:ai2- ---ii:iii2S- -----trace

~r:~f!-----~_~~_~-~::: _::;-~~.~ ::::~: :j:~~~ ----l:

from ~he surface of the shells.
Chemical composition of shells of O. edulis is
not significantly different from that of C. virginica.
Table 8 gives the results obtained by European
scientists. The data quoted from various sources
are taken from Vinogradov (1937). According to Creac'h (1957), all shells of O.
A much more detailed analysis of dead oyster edulis and C. angulata contain traces of phos-
shells dredged from the bottom of Galveston Bay phorus. The French biologist found that the
8 miles east of San Leon was made recently by the phosphorus content is variable. Expressed as
Dow Chemical Company (Smith and Wright, P 205, it varies in C. angulata from 0.075 to 0.114
1962). The shells were scrubbed in tap water percent. There is a significant difference in the
phosphorus content in various parts of the shell.
with a nylon brush, rinsed in distilled water, dried
at 110° C., and ground in a porcelain mortar. The amount of phosphorus per unit of volume of
shell material is lower in the chalky deposits than
With the kind permission of the authors the
in the hard por~ion of the shells. Thus, in laying
results are given in table 7. Additional 19 ele-
a chalky depOSIt the mollusk utilizes from 2.4 to
ments were sought but not found at the following
2.6 times less phosphorus than is needed for
sensitivity limits:
secreting the same volume of harder shell
10 p.p.m.-arsenic, barium.
substance.
1 p.p.m:-antimony, chromium, cobalt, ger-
The presence of small quantities of strontium
mamum, gold, lead, lithium, mercury,
~n calcareous shells of mollusks is of particular
molybdenum, nickel, vanadium, and
lI~terest because. of its apparent relation to arago-
zirconium.
mte. The marIne organisms containing calcium
0.1 p.p.m.-beryllium, bismuth, cadmium,
carbo~ate as aragonite have relatively higher
silver, and tin.
strontIUm content than those having calcite shells.
The authors remark that traces of clay entrapped
The relationship between the two elements is
within the shell may have influenced the findings
expressed as strontium-calcium atom ratios
for titanium, manganese, copper, or zinc; and
(Tho.mpson and Chow, 1955; Trueman, 1944; and
that individual variations in silicon, iron, and
Asan, 1950). In C. virginica and C. gigas the
aluminum were due to contamination not remova-
strontium-calcite ratio x 1,000 varies between
ble by washing. It appears feasible that these
1.25 and 1.29. Ostrea lurida from California has
variations may have been caused by spicules of
a lower strontium content, the ratio being 1.01.
boring sponges and algae infesting the shells.
!he percentages of Ca, Sr, CO2 , and organic matter
TABLE 7.-Composition of C. virginica oyster shell dredged In the ~hel~s of t~ee species of oyster and in Mya
from Galveston Bay, accordmg to Smith and Wright arenaNa, In whICh the content is the highest
(1962)
among the bivalves, given by Thompson and
Constituent Concen· Constituent Concen·
Chow (1955), are summarized in table 9. The
tration tratlon
TABLE 9.- The percentage of calcium and strontium in the
Percent P.p.m. shells of oysters and soft shell clam
Calcium (CaO). _ 64.6 Organic Carbon as CH.__ 400
Carbon (CO,'._. _ 43.5 Chlorine (Cll- _ 340 [According to Thompson and Chow, 1955]
Sodium (Na'Ol .• -------- 0.32 Aluminum (All---------- 200
Maf.neslum (MgOl.----- 0.33 Iron (Fel . _ 180
Sui ur (SO,'---------.--- 0.16 Phosphorus (Pl ..-------- 116 Atom
Silicon (SIO,'------------ 0.16 Manganese (Mnl.------- 110 Species Calcium Strontium Carbon Organic ratio
Strontium (SrOl-.---.--. 0.12 Fluorine (Fl-- __ 54 dioxide matter Sr/Ca
Moisture (H,Ol---------- 0.58 ~~:llS!!lum (~l-----------. 30 xl,OOO
anlUm (TIl-- 12
Total of major con· Boron (Bl---_. _ 5
stituents.. _.. __ % Copper O.lurida_. ______ 38.6 0.085 42.5 1.01
99.8 Zinc (Znl(Cul-----------.-
__ 3
2 C. virginica______ 33.7--37.8 0.92-0.107 41.8-42.4
1.68
2.16-2.34 1.25-1. 29
Bromine (Brl _ 1 C. gigas _________ 34.6--36.2 0.097-0. 100 32.6-42.5 1. 33-1. 71 1.26-1. 28
Iodine (1)---------------- 0.5 M. arenaria _____ 38.6-38.8 0.181-0.246 42.2-42.3 2.22-2.44 2.16-2.91

44 FISH AND WILDLIFE SERVICE

salinity and temperature of the wat~r have ~p­ des Seances de l' Academie des Sciences, tome 203,
pp. 965- 968.
parently no influence on Sr/Ca, WhICh rem~ms
FREMY, E.
fairly constant in calcareous shells. The possIble 1855. Recherches chimiques sur les os. Annales de
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788-851 0-64-4
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