ARY

IS1TY OF :ORNiA

MURICIN^E.

PLATE 1.

MANUAL

OF

GEOLOGY;

STRUCTURAL AND SYSTEMATIC.

WITH ILLUSTRATIONS OF THE SPECIES.

BY GEORGE W./TRYON) JR.

/

CONSERVATOR OF TUK CONCHOLOGICAL SECTION OK TUK ACADEMY OF NATURAL SCIENCES OF PHILADELPHIA.

VOL. II.

MURICIN^ PURPURINJE,

^4y /

4*4 f

PHILADELPHIA: Published by the Author,

ACADEMY OF NATURAL SCIENCES, COR. ISTH & RACE STS.

1880.

SCIENCE! ySjj

LIBRARY

WM. P. KILDARE, PilNTER, 734 & 736 SANSOM ST , PHILA.

IN THE preparation of the introductory portion of the present volume free use has been made of Keferstein's admirable work, forming part of" Bronn's Klassen und Ordnungen d. Thierreichs:'' — the most comprehensive treatise on the anatomy of the mollusca which has }^et appeared. Of course many additional facts and some corrections have been published since the issue of that work, and abstracts of the most important of these have been herein incorporated. I have endeavored to avoid the prolixity of Keferstein without sacrificing clearness of language, but occa- sionally quote his very words. The few statements for which I alone am responsible, are made in the first person. The body of literature consulted is too extensive for quotation in a work like this, where much must necessarily be sacrificed for the sake of brevity. Whilst 1 have freely used the admirable illustrations of Keiner, Reeve, Sowerby, Kiister, and other monographs. I have added to these hundreds of figures, names and items of infor- mation gleaned from fugitive papers and scientific periodicals ; so that I am enabled to present a ca refully arranged, concise account of all that is known concerning the Murices and Purpurte.

I may be thought 03- some, to have treated the species in a too conservative spirit; my object in the investigations of which this work is the result, has been solely to ascertain truth, not to establish, maintain or destroy scientific reputations.

January. 1880. (T. W. T.. JR.

If there be nothing new, but that which is, Hath been before, how are our brains beguil'd,

Which, laboring for invention, bear amiss, The second burden of a former child !

SHAKSPERE.

LIBRARY

UNIVERSITY OF]

PROSOBRANCHIATA.

ANATOMY.

External Features.

The body in prosobranchiates presents the following, more or less, differentiated portions :

1. Head. This is sometimes partially separated from the body by an intervening- constriction or neck, but is very 'frequently only indicated by the possession of a mouth on its ventral and of a pair of tentacles on its dorsal surface. From the latter sometimes proceed pedicels bearing the eyes.

2. Mantle. This covers the posterior portion of the dorsal surface of the body.

3. Foot. Arising from the ventral anterior part of the body.

The mouth may be either simple, in the plane of the head, or it may be produced at the end of a contractile rostrum, or of a retractile proboscis. Behind the head and on the under side of what ma}^ be called the neck is attached the foot, and behind the neck begins the posterior portion of the body covered by the mantle : this part of the animal is Arery long, usually, and as the shell is moulded upon it, it assumes a similar form — general!}" spiral. However, in Patella and similar limpet-like shells, the body is short and straight. The mantle encircles the body like a collar below, but above it is much produced over the back of the animal, and the respiratory cavity forms a sinus of its margin. The foot consists of the attachment to the neck, which is usually narrow, and an expanded portion or sole, or creeping disk. Such is its typical form, but in Patella the foot embraces the entire ventral surface of the animal. It is a muscular pro- cess of the body, and the only locomotive organ possessed by prosobranchiates.

('» EXTERNAL FEATURES.

Viewed externall}7, the prosobranchiates, as well as the mol- lusca in general, must be regarded as bilaterally symmetrical ; a view which is strengthened by the position of the nervous system ; and the developmental history of the intestines, though its actual position, as well as that of the sexual organs, does not correspond with this symmetry. Thus we find that at first the anus is at the posterior end of the body, but gradually it ap- proaches nearer and nearer to the anterior end until finally it opens on the back or right side near the mouth. In its early stages the shell and mantle only occupy the extreme posterior end of the body, but in their growth they cover more and more of the latter, gradually pushing the anus forward.

An external shell, usually sufficiently large to contain the entire animal, is common to all the prosobranchiatee. It is a secretion of the mantle, and conforms to its shape; and the hard- ness which it assumes by the addition of carbonate of lime, renders it an efficient protection to the animal, whilst the faith- fulness with which it reproduces the external features of the latter renders it extremely useful in classifying the niollusca. The spiral growth of shells is as nearly of true mathematical regularity as is possible in an organic body, forming the loga- rithmic spirals of Moseley. or conchospiral of Naumann. Cor- responding to the shell, which is produced by the mantle, and borne by the posterior portion of the body, the posterior dorsal part of the foot bears an operculum, secreted by an expansion of its skin called the opercular mantle. Typically, the operculum is a spiral also, but in the same plane ; yet in many cases its growth is annular. Usually horny, it is sometimes nearly, entirely or partially calcareous, and on the retirement of the animal within the shell it is brought into the aperture of the latter, which it more or less complete!}7 closes. Like the shell itself, it may be considered a protective organ, and when in apposition with the former suggests the two enclosing valves of the lamellibranchiate or bivalve mollusks.

Notwithstanding the large portion of the animal which is always within the shell, even when the head and foot are ex- truded, the latter is only attached to the former at one point, on the columella, and by means of a columellar muscle, which, pass- ing through the foot, is attached at its other end to the oper-

THE SKIN. 7

culum (when the latter is developed, which is not always the case . Although there is only this single actual bond of con- nection between shell and animal, the contact of the body serves to maintain the vitality of the shell, which soon bleaches, and finally decays when separated from its architect and inhabitant. We will now describe, in a necessarily cursory manner, successively, the skin, the foot, the muscles, the shell and operculum, the digestive, nervous, vascular, respiratory, secre- tive and sexual organs of the prosobranchiates.

The Skin.

The body is completely enveloped by MH external skin. Its epithelial layer is formed of quadrangular or prismatic cells, which have a distinct nucleus, and occasionally, when long- cylindrical in form, they have a tail-like end beneath, penetrating the cutis.* Externally the epithelium sometimes supports cilia upon the exposed portions of the body.

The cutis consists essentially of tine interlacing muscular fibres with interposed cells; often attaining a perceptible thickness. The subcutaneous muscular layers of the body are immediately continuous with those of the cutis: the fibres of which they are formed may be clearly distinguished as an outer longitudinal and inner circular layer. These fibres are i as in molliisks gene- rally) compressed and band-like, with pointed ends and central oval nucleus.

The skin forms a fold above and surrounding the foot, and this portion is technically termed the mantle; it surrounds the body, behind the cephalic portion like a collar, and thence spreads dorsally over the posterior part of the animal. In the terminal, thickened border of the mantle, the cutis becomes of greater thickness ; its upper stratum containing very numerous glands, furnishing the mucus and colors which are here mingled with the secreted shell-material. Similar glands, furnishing a copious supply of mucus are found also in the cephalic portion of the body as well as in the foot, and especially in the sole ; but no cells are found in that part of the animal permanently covered

* It is possible that these last are epithelial terminations of nerves— hence sense organs.

8 THE SKIN.

by the shell. Frequently the external skin is colored by a gran- ular pigment, which either is contained in cells, lying between or enveloping the glands, or else sometimes appears to lie free under the epithelium.

The mantle border is the principal agent in the secretion of the shell : it is thrown out by the epithelial layer as a sort of cuticular development. With the organic basis of this secretion is mingled carbonate of lime, originating in the epithelial cells, where it may be separated from the blood : in hardening, the exuded material becomes half crystalline or laminated. Usually the external layer of the shell is a transparent or translucent skin, the epidermis ; having no lime in its composition. It is often colored by pigments lying in the outermost border of the mantle.

Whilst the growth of the shell is thus provided for by addi- tions of the aperture margin from the mantle border, the whole mantle is equally capable of producing shelly substance ; and not only are shells thus thickened from within by the mantle surface, but breaks are repaired with new material by a similar provision : only such repaired and interior portions are devoid of epidermis and of color, the pigments being found only in the free border of the mantle. Mollusks are even able to secrete shelly matter to provide against threatening- dangers from the boring of other animals into their shell.* In certain genera, as Haliotis, Turbo, etc., the inner shelly layer is nacreous — that is, it forms mother- of-pearl.

The mantle border by means of its sphincter muscles embraces the body closely, thus closing the mantle cavity except at one point, where a small opening allows the ingress and egress of water for respiration. This respiratory opening is a semi-circular notch, formed by muscles, and is sometimes prolonged on its dorsal wall into a half-closed tube or respiratory siphon; which, when present, assists by the phases of its development in the classification of the mollusca. This siphon usually forms an an- terior notch in the shell near the margin of the columella and the

* A curious example of shell secretion by the visceral mantle occurs in a cone belonging to the cabinet of the late Dr. Gray. A section of this shell has been made, showing holes bored into the spire end by lithodomi and the repeated walls erected by the animal across the ends of the whorls to protect itself against the ravages of its insidious enemies, (plate 2, fig. 1.)

THE SKIN. 0

existence of the latter thus predicates that of the former. The siphonal tube is sometimes greatly prolonged, and is then fre- quently covered for most or all its length by a prolongation of the aperture, which is technically known as the canal of the shell. The canal in Mu rex and Pusus is extremely long, at least in the typical species. Mollusks of which the shells are furnished with a canal or anterior notch are called siphonostomata, the first great division of the prosobranchiates. The siphon is principally confined to predatory or carnivorous mollusks. The second great division, termed holostoniata, have rounded apertures ; consequently no siphon but simply an opening for respiration. They are vegetable feeders usually (Natica is a remarkable ex- ception), and close the aperture of the shell completely by their operculum.

At the posterior left border of the mantle, behind the branchht, is sometimes an opening from which a small siphon extends back- wards, and when it is present, it forms a notch in the posterior part of the shell, as in Cypraea and Conus, or a canal as in Ovula, or frequently it only forms a callosity on the upper part of the columella. close to its junction with the posterior part of the aperture margin. Probably this siphonal opening is for the exit of the water that has entered lay the branchial opening. In many of the siphonostomata it is not present.

The mantle border can be freely withdrawn within the whorl, as it is not united to the shell at any point. It is frequently- prolonged into digitations, or exhibits prominences or invagina- tions, all of which develop similar features on the shell ; thus giving rise to the fingers of Pteroceras, the spines of Murex, etc. Occasionally, howrever, processes of the mantle do not secrete shelly coverings : Cerithium and the oriental Melanians, for in- stance, have delicately digitated mantle margins, these digita- tions forming no secretion, and sometimes thrown back over the shell.

The mantle is occasionally largely developed into side lobes, which in Marginella and Cypraea are so extended as to be thrown up over the external surface of the shell, nearly or completely covering it. In such shells an epidermis is not present. The mantle lobes of Cypr*ea are beset with numerous papillae, which seem to partake the function of tentacles as tactile organs. In 2

10 FOOT.

other genera, as in Oliva, the mantle is prolonged into filiform processes before and behind.

The female Vermetus has the mantle cleft in the middle, ac- cording to Lacaze-Duthiers, although there is no corresponding cleft in the shell, and in Haliotis a similar mantle cleft impresses a groove in the shell, in which are situated the row of holes characteristic of the genus. The shell of Pleurotoma also has a sinus corresponding to a cleft mantle. The cause of the sutural sinus of the shell of the American fresh-water genus Schizostoma is as yet unknown; it may be due to a similar cause or it may be sexual. As the genus is restricted to the Coosa River and its neighborhood, I am inclined to think that it is a local disturbance of growth, especially as most of the species could not be distinguished from corresponding forms of Gonio- basis except by the lip notch or slit.

Foot.

The foot is a fleshy, expanded mass, attached to the under side of the body, in front of the mantle by a peduncle. In the heteropods the foot is divisible into three portions, termed, respectively, propodium, mesopodium and metapodium ; but in the typical gasteropods these three areas are blended in the sole, although the metapodium is indicated by the fact of its support- ing on its dorsal side the operculum. In Strombus (pi. 3, fig. 14), a transverse furrow separates the mesopodium from the propodium, and the metapodium is covered downwards and in front by the operculum.

The peduncle of the foot is usually short and depressed", and covers the under side of the body between the mantle collar and mouth, the foot being expanded forward, but more extensively backwards ; but in Strombus and its allies the operculum is long and narrow, whilst the foot is slim and cylindrical. Haliotis, Patella and Chiton have the foot, on the other hand, very much expanded. Rapidity of motion appears to be in inverse ratio to the size of the foot ; those genera in which this organ is enor- mously developed, especially in those just cited, where it occu- pies the entire ventral surface of the body, being slow in movement.

FOOT. 11

Generally, the anterior border of the foot is variously lobed (plate 3), and these lobes are mostly of glandular structure, though Keferstein has found them in Buccinum to consist of interwoven meshes of muscular fibres, in which are lodged beau- tiful nucleated cells. These lobes or filiform processes, like those of the mantle, are tactile organs.

When the foot is greatly expanded, as in Cymba, Harpa, Doliuin, etc., it is generally laterally recurved over the shell, somewhat like the mantle in Cyprsea and Marginella. In Oliva (pi. 3, fig. 16) and in Ancillaria, the propodium, repre- sented by triangular lobes, lies flat, while the distinctly separated mesopodium covers a portion of the sides of the shell. In Natica the propodium is greatl}' developed forwards, extending beyond the head and reflected backwards over the latter in such manner as to conceal it, with its tentacles, and the anterior part of the shell itself, from view. The mesopodium in Natica, being like- wise largely reflected over the shell, the respiratory cavity is covered, but a canal is developed between the reflected pro- podium and the mesopodium, which conveys water to the branchiae.

There is a rounded glandular opening in the sole of the foot of many prosobranchiates (Pyrula, pi. 3, fig. 18), which is the external opening of the pedal aqueous vascular system ; to be more particularly noticed hereafter.

Nearly all of the prosobranchiates whilst in the larval state support an operculum on the dorsal side of the metapodium ; with some, the operculum is fugatious, and is lost in the trans- formation of the animal, yet it continues present in most of the groups. Its secretion from the operculigerous mantle, and its morphological relation to the shell, will be explained further on. The opercular mantle sometimes extends beyond the borders of the operculum itself, and is divided into processes or filaments, which may, as in Ampullaria, be reflected over it.

The foot is the organ of locomotion of univalve mollusks : the anterior portion is protruded, and then by means of wave- like muscular contractions of the sole, the whole mass of body and shell is brought forward — when the former motion is re- peated. In Phasianella, Quoy and Gaimard have observed that the foot is divided lengthwise by a furrow, and that when motion

1 '2 COLUMELLAR MUSCLE AND OPERCULUM.

is rapid the muscles of the sole are alternately used on either side, so that the effect of the motion is that of a pair of feet. In attached shells, like Vermetus, the foot is only rudimentary and serves merely as a support to the operculum.

In retiring within the aperture of the shell the foot is generally doubled upon itself across the middle, so that its dorsal posterior side, bearing the operculum, comes outermost ; but in Oliva and Voluta it folds longitudinally, whilst the quadrate foot of Conus is withdrawn obliquely, without folding, first the right and then the left side.

The Columellar Muscle and Operculum.

As already stated, there is but one attachment of the proso- branchiate to its shell ; namely, by means of the columellar muscle, by which the inner face of the columella is directly united with the posterior portion of the body of the animal. It passes underneath the mantle, greatly thickening the body wall, and terminates upon the inner face of the operculum, so that by its contractions the operculum and shell are approximated. The form of this muscle depends on that of the shell, and in the conical, non-spiral shells especially, varies greatly from its normal development. Thus, it is horseshoe-shaped in Capulus ; it is divided into two portions, one of which lies on either side of the anterior part of the animal, in Fissurella. In Haliotis the animal is coiled around it, and its insertion, instead of being on the columella, is on the middle of the inner wall of the shell itself.

At the ending of the columellar muscle in the dorsum of the foot, its fibres are nearly vertical to the plane of the operculum, which usually appears to be immediately superimposed upon them : in Buccinum, however, Keferstein finds interposed a layer of long cylindrical epithelial cells, with mostly distinct nuclei, and long divided processes entering between the muscular fibres.

The operculum, a cuticular development of these cells, is com- posed, as may be seen in the corneous opercula of Murex, Purpura, Triton, etc., of very thin superimposed layers. With the microscope one may perceive in a thin section, the cylindri- cal cells with their head attached to the lowermost layer ; or, on

COLUMELLAR MUSCLE AND OPERCULUM. 13

the inner face, the small rounded pittings where they have been attached.

Dr. J. E. Gray was the first investigator who announced that the operculum is homologous with the second valve of the lamelli- branchiates or bivalve mollusks. He has shown that the oper- culum is developed on the embryo long before it is hatched ; that it is placed on and covers a particular part of the body called the lobus operculigerus, and which bears to it the same relation which the mantle does to the shell, and that its growth occurs in the same manner ; that this growth is made by the ad- dition of new matter to the inner surface and especially near the margin ; that it is attached to the animal by means of one or more muscles, which, as in the bivalve shell, pass from the larger valve or shell to the smaller one or operculum ; that the operculum, as it increases in size, is gradually moved on the end of its muscle — the many-whorled operculum of the Trochi re- volves as many times on the end of the muscle as the many- whorled spiral shell turns on its imaginary axis ; that the opercu- lum is often lined internally with a shelly coat like a shell, and sometimes, like the Cowries, its outer surface is covered also with a shelly deposit by a special development of the opercular lobe.

The principal difference between the operculum and the valve or shell of the Gasteropods consists :—

1. In the operculum having no cavity, its cone being de- pressed, flat or even concave, or very much compressed, form- ing only a spiral riband, as in the spiral operculum. But this absence of a cavity is a difference only of degree, for the valves of some Gasteropods, as Umbrella, Patella, etc., are much flat- tened ; the first resembling the annular operculum of Ampul- laria and Paludina : but the greatest resemblance is to be ob- served in the small, flat valves of Gryphaea, Exogyra, Chama, and other genera of bivalve shells which are attached by one of their valves. These valves are often quite as flat and destitute of any cavity as the operculum of any Gasteropod ; and it is to be remarked that these valves exactly resemble a spiral opercu- lum in shape, the remains of the ligament forming a spiral mark on the outer surface, showing how the valve has rotated on the body of the animal as the operculum rotates on the foot of the Gasteropods.

14 COLUMELLAR MUSCLE AND OPERCULUM

2. The operculum is generally horny and formed of a sub- stance similar to the epidermis of shells ; but then some shells, like the Bulla, Aplysia, certain Uniones, etc., are entirely or almost destitute of calcareous matter, and some of the helices, when inhabiting granitic regions, are equally of epidermal sub- stance : on the other hand many opercula are thickened inter- nally with a calcareous deposit.

Dr. Gray proceeds to show that in bivalve shells like Chama, where one valve (the attached one) has a spiral apex, whilst the other valve, is a flattened spire, the position of the hinge with reference to the spire must rotate slowly with growth, as in the spiral operculum in its growth. The direction of the spire of a spiral operculum is opposite that of the shell, showing another analogy.

The conclusion arrived at by Dr. Gray is that the normal or typical form of mollusks is that protected by two valves or shells : indeed, some nudibranchiate gasteropod mollusks which have no shell in their adult state (Doridse), have their newly hatched young covered with two shelly valves which afterwards fall off.*

Dr. Gray has always maintained that the opercula are of great value in the distinction of genera, and he does not fail to condemn severely the practice of preserving shells in museums, or of figuring and describing them in conchological works, with- out opercula. I have already alluded to the supposed opercular bodies found with the fossil Ammonites : f it ma}' be added that they occur as well in some of the Heteropods and Fteropods.

In 1847, Loven proposed to consider the operculum as analo- gous to the byssus ; but Dr. Gray has pointed out that some genera of Gasteropods provided with an operculum, secrete a byssus also (Rissoa, Cerithium, Littorina, etc.). However, Prof. Huxley, one of the latest and best authorities, thus endorses Loven's views :

" On the haemal aspect of the posterior portion of the foot, a chitinous or shelly plate, termed the operculum, may be de- veloped. This operculum appears to be the analogue, if not the

* Ann. Mag. Nat. Hist., 2 ser., v, 476, 1850. f MAN. OP CONCH., vol. i, Cephalopoda, 267.

COLUMELLAR MUSCLE AND OPERCULUM. 15

homologue, of tlie byssus of the lainellibranchs ; and is certainly not homologous with either of the valves of the shell of the latter, which are pallial structures." *

The following interesting note by Dr. Gray, will throw some light on this very interesting subject :

" Oti the reproduction of the lost part of an operculum., and its probable restoration ivhen entirely destroyed.

" It is to be expected that the operculum of a gasteropodous rnollusk may be sometimes broken or injured, but I have never hitherto been able to find any very distinct example of the kind, so as to study how the repair of the lost part would be effected. That such an occurrence would most probabty be rare, is easily explained from its situation, as the operculum is protected by the last whorl of the spire of the shell when the animal is expanded, and by the mouth when it is contracted into the cavity of the shell.

" I have lately met with a very distinct example in a specimen of Fusus in the British Museum collection. In this specimen the apical half of the operculum has been broken off (pi. 2, fig. 12 a), and the lost part has been renewed by an irregular round- ish process, nearly of the size of the lost part, not quite as thick as the original portion, and formed of rather irregular horny plates ; the smaller or first-formed portion being in the centre of the broken line, so that the restored part bears some similarity to the annular operculum of a Paludina. This restoration is exactly like that which would have taken place in a shell under similar circumstances, and is a further proof of the truth of the theory which I have long advocated, that the operculum is a rudimentary valve, and is homologous to the second valve of the bivalve mollusks.

" In examining two specimens of Pleurotoma babylonica, pre- served in spirits, with the opercula attached, I was much sur- prised to observe that the opercula of the two specimens were exceedingly different in structure and belonged to two distinct modifications of that valve, one (pi. 2, fig. 12 b) being sub- annular, with the nucleus apical, like the other species of the genus, and the other (pi. 2, fig. 12 c) annular, with the nucleus

Huxley, Anat. of Invert., 487, 1877.

16 COLUMELLAR MUSCLE AND OPERCULUM.

sub-central, somewhat like the operculum of Paludina. Tlie examination of the restoration of the lost half of the operculum of the Fusus before referred to has solved the difficulty, and I have no doubt that one of these animals had by some accident lost its operculum, and that it had gradually restored it; com- mencing, as in the case of the restored part of the operculum of the Fusus, by a small nucleus in the centre of the opercular mantle, on the back of the foot, and gradually adding new Ia3^ers around the edge of it, until it formed an annular operculum nearly of the size of the original, but differing from it in shape, being less acute in front and nearly similar in form at the two ends. A more minute examination has strengthened this theory, for the operculum of this specimen is less regularly developed than is usual in the annular operculum of the kind, and is much thinner than the normal operculum of the genus, as is the case in both these particulars with the restored part of the operculum of the Fusus.

u This change in the formation of the operculum when it is reproduced is just what might have been expected. The animal, when it has to form the operculum at its birth, begins its forma- tion at the tip, and increases its size, as the animal requires a larger operculum for its protection, by the addition of new layers to the outer edge of its larger and last-formed end ; but when it has to reproduce this organ, the opercular mantle having reached a certain size, it proceeds to cover its surface with a new pro- tection in the most easy and rapid manner, and, commencing from a more or less central spot on the surface, enlarges the surface covered by adding new matter to the entire circumfer- ence of the first-formed part ; it continues this process without waiting to making the operculum as thick and solid as the one which was lost, until it readies the size of the original, moulding itself on the opercular mantle, and adapting its form to the form of the throat of the aperture of the shell which it has to close. The change of form in the front of the restored and mended operculum is caused by the parts being moulded on the existing opercular mantle — consequently they have not the narrow front part which is found in the normal form caused by that part having been formed when the animal had this part of a small size; and as it increases in size the whole opercular mantle

THE SHELL. 17

moves forward, leaving the small tip of the operculum free, and useless to the animal, and, therefore, not necessary to be repro- duced when the operculum is reformed in the adult age of the animal."*

The Shell.

All prosobranchiates (and nearly all mollusks) are provided with an external shell, a dwelling place and a citadel combined, the hardness and durability of which, as Keferstein remarks, " supplies us with the best means of knowing the animal ; in- deed, in many cases, it is the only part known, and was formally the only part valued and preserved in collections. Although the animal itself offers more weighty and striking characters for the separation of the higher groups, yet having learned the close relationship existing between shell and animal, we find therein ample justification for attaching especial importance to the shell in a systematic point of view."

We have already shown how the shell is produced by the mantle.

The form of the shell is throughout regular, and is normally a cone curved into a spiral, and descending in a screw-like manner from the apex or initial whorl to the aperture. Nothing can be more beautiful than the regular geometrical progression of the growth of a shell or the certainty with which each species and genus grows in its normal pattern, although these modes vary among themselves so widely : thus we have the simple depressed cone of the Patella, all aperture and no spire, and from it every gradation from the Haliotis, almost equally depressed and broad, the result, however, of a very rapidly enlarging spiral, to the long, many-whorled Turritella, or the Vermetus, which is a Turitella partially unrolled into a simple long tube : — the opposite of the Patella. The whorls of a spiral shell are, in most cases, closely wound around its axis, and, therefore, most part of their surface is in contact, each whorl partially covered and concealed by its successor ; and where the axis does not lengthen by the obliquity of the spiral, we find, as in the cone and Cypraea that the shell only shows externally its last whorl, with, perhaps, a

* Dr. J. E. Gray, Proc. Zool Soc., 100, 1854. 3

18 THE SHELL.

very small portion of its predecessor visible on the spire. On the other hand there are genera in which the whorls are not at all in contact, and where the axis becomes itself an imaginative cone, widest at the base. Besides the almost numberless modifi- cations of form resulting from the degree of obliquity and close- ness of the spiral, the direction of the latter may be mentioned as another factor in producing modification. In most spiral shells the spire normally curves to the right, that is to say, plac- ing the shell with its apex turned from the observer and its aperture in view, the latter will be found on the right hand. In others the volutions proceed in the opposite direction with such regularity as to be eminently characteristic of some species and genera. However, in certain genera, it is found that species normally dextral will exceptionally produce sinistrally coiled shells, and this abnormal growth probably is caused by disturb- ance of the relations of the embryo with its initial shell.

Whilst the bulk and weight of shells are composed principally of carbonate of lime, yet they have always an organic basis, which is first developed, and then gradually impregnated with the lime. If the latter be removed by the use of acids the organic residuum (conchy olin) still retains the shape of the shell, forming a sort of membranous framework. It is this organic basis which maintains the life of the shell, for, the animal re- moved, as in beach-worn or fossil species, the conchyolin soon disappears and the shell becomes pure carbonate of lime, growing at the same time more and more brittle. Many of the long spiral shells, such as Bulimus decollatus, some Pupas, Truncatella, Melania, etc., withdraw the body from the earlier whorls in the course of growth, and partition off the unoccupied space with a shelly plate : in such cases the unoccupied whorls become brit- tle and are soon broken off. We must, therefore, believe that the shell is vitalized, or rather, that its vitality is maintained by simple contact of its organic basis with the living animal.

There are two very distinct types of shell structure, the cel- lular or porcellanous and the membranous or nacreous. In bivalve mollusks the former is the outer and the latter the inner layer, in most cases, but in univalves the shell is usually porcel- lanous only ; although a few of the holostomata, such as Turbo,

THE SHELL. 19

Troclms, Stomatia, etc., have both layers, the inner one pearly.

Nacreous shell substance consists of very thin, superimposed membranes, lying parallel with the surface and impregnated with aragonite. The iridescence of this substance proceeds from the waved or wrinkled close edges of these membranes. The aragonite is of crystalline structure, showing beautiful five or six angled tables. (PI. 2, figs. 8, 9.)

As porcellanous shell structure is essentially the same in almost all Gasteropods, and the only kind of shell structure in most of them, a few sections of Strombus gigas will give a clear idea of the disposition of the material. (PL 2, figs. 4-6.)

There are, it will be perceived, three layers of deposition (of which the middle one is thickest in this case), each composed of a multitude of plates or prisms, but each differing in the direc- tion of arrangement of these. It will be readily perceived how much this diversity of arrangement adds to the toughness of the shell structure, as no line of fracture can penetrate the entire shell wail, except by the violent breaking across of part of these layers of prisms.

As to the chemical composition of shells, the conchyolin or organic material is a small, varying percentage, carbonate of lime, existing in quantities varying, from Turritella, 88.70 per cent, to Strombus gigas, 99 per cent. There are traces of other constituents, of which carbonate of magnesia is perhaps one of the most important ; it varies from 0.12 per cent, in Teles- copiuni to 0.48 per cent, in Fusus antiquus. Silicic acid has also been detected. C. Schmidt has obtained almost 1 per cent, of phosphate of lime from the shell of Helix nemoralis. The calcareous operculum of Turbo (analyzed by Wicke) con- tains : Carbonate of lime, 98.72 per cent. ; organic material 1.28 per cent.

Shells are perceptibly harder than, and will scratch calc-spar. Their specific gravity is somewhat higher than that of Carrara marble, being about 2.15 to 2.85 for the prosobranchiates, and varying but slightly in the pulmonates.

With reference to the coloring of shells (terrestrial as well as marine species) Fischer points out that as a rule, brilliancy of coloring increases towards the equator. He suggests three

20 THE SHELL.

zones of coloration corresponding to the thermal zones, which may be designated as :

1. Monochromic or frigid zone.

2. Oligochromic or temperate zone.

3. Polychroniic or tropical zone.

Of course exceptions are numerous — modifications based upon habits, ocean currents, adaptive coloration, etc.. M. Fischer particularly cites the melanism which characterizes so many shells of the West Coast of North and South America, giving, among other examples, the numerous species of sad colored and lugubriously named Trochi, which pervade those regions.*

On adaptive coloration. — A number of papers on mimicry or adaptive resemblance as a means of protection of animals against their enemies have been published, and the subject has awakened much interest and discussion. Coincidence of colora- tion of molluscous animals with their surroundings has been frequently remarked, but it is probably not so often for purposes of attack or security as the result of the food upon which the^y live or a dye obtained by contact with it. I have shown in my volume on the Cephalopoda that conscious adaptation of color has been noticed in the Octopus. Mr. Morse gives a number of instances of coincidence of color between American marine mol- lusks and their surroundings which he supposes to be evidences of protective adaptation.f

" The thickness, the roughness, and the smoothness of the sur- faces of shells appear to depend, in a great measure, on the still- ness or agitated state of the water in which they reside. Shells which have branching or expanded varices. like the Murices, are also much influenced by circumstances, and hence many mere varieties, arising from local causes, have been considered as dis- tinct species. Thus Murex anguliferus is merely a Murex ramo- sus with simple varices ; and Murex erinaceus, M. torosus, M. subcarinatus, M. cinguliferus, M. tarentinus, and M. polygonus are all varieties of one species. Murex magellanicus, when found

* Dr. P. Fischer, Jour, de Conch., xxiii, IQ.% 1875.

f E. S. Morse, Proc. Bost. Soc. Nat. Hut., xiv, 141, 1871.

SHELL. 21

in smooth water, is covered with large acute foliaceous expan- sions ; but the same shell living in rough seas is without any such expansions, and only cancellately ribbed. In such situa- tions it seldom grows to a large size ; but when it does so, it becomes very solid, and loses almost all appearance of cancella- tion. Triton maculosus is very widely spread over the ocean in different temperatures and different kind of seas ; it consequently offers a multitude of varieties both of size and surface, all gradu- ally passing into each other, and most probably produced by the operation of the foregoing causes. Indeed, a vast number of merely nominal species have been formed from the habit, too prevalent among conchologists, of describing from single speci- mens, or even from several individuals brought from the same locality, which would never have been considered as distinct had collectors kept in their cabinets a series of specimens found under different circumstances, and studied, on the coasts where they are found, the variations which shells undergo."*

Shell.

Rest periods in the growth of mollusks are sometimes, as in the Murices, marked by a thickening of the edge of the shell, caused by continuous depositions of shell material, forming a ridge or varix ; and the rate of growth may be thus traced readily, in numerous mollusks. Thus in Murex one group shows three varices upon each whorl, indicating that a period of three years, or at least three seasons of activity is required for the completion of a single whorl of growth. In another group of Murices the varices are more numerous, as many as four to ten being counted on a whorl. In Triton, the varices are two on each whorl, but nearly alternately situated, so that the varices of each whorl oc- cupy an intermediate position to those on the preceding whorl : in Ranella there are also two varices but they form a continuous fringe or wing-like expansion on each side of the spire, showing a very regular growth by periods of half-whorls.

The accretion of surface during growth is not continuous but is made by minute layers, around the margin of the aperture, each extending a little beyond its predecessor, and the edges of

Dr. J. E. Gray. PMlos. Trans., 771, 1833.

22 SHELL.

these layers as exposed on the external surface are called growth- lines. Those shells which have a simple, or sharp-lipped aper- ture and which do not develop varices, nevertheless distinctly show the rest-periods by the greater impression of the growth- lines. Many shells retain the sharp aperture for a variable period, which may be called their juvenescence, but finally acquire adult characters, consisting of a thickened, reflected, inflected or lipped aperture — which is sometimes more or less contracted by inflected calcareous projections called teeth. Growth, however, frequently continues after adult characters have been acquired, and then these are absorbed away when accretion recommences, leaving the mark of their former position in a more prominent growth- scar or line.

The power of dissolving their shells is possessed by certainly a large portion of the mollusca and is habitually exercised by many of them : thus the cone, which we have seen partitioning off its whorls against an enemy and thus seriously incommoding itself for room, would under normal circumstances acquire for itself additional accommodation by absorbing away most of the thickness of the enclosed whorls or partitions, and Cypraea, Nerita and Auricula assist themselves in the same manner so as to become eventually an external shell only, with a single cavity. Tn species with lengthened spire, this method is not so practi- cable, because only a small portion of the whorls are enclosed within succeeding ones. The growth of many of these animals is such that they cease after awhile to occupy some of the earlier whorls? and they then partition them off as a regular habit, in the same manner that the cone has worked under the pressure of excep- tional necessity. These partitions made, the portion thus cut off' from contact with the animal loses vitality, becomes brittle and breaks off, forming the truncated shells which are characteristic of numerous groups — especially of land shells.

From what has