  A CONCHOLOGICAL MANUAL. BY G. B. SOWERBY, Jun. ILLUSTRATED BY UPWARDS OF SIX HUNDRED AND SIXTY FIGURES SECOND EDITION. CONSIDERABLY ENLARGED AND IMPROVED. LONDON: HENRY G. BOHN, YORK STREET, COVENT GARDEN. MDCCCXLII. PREFACE TO THE FIRST EDITION. It may be necessary in introducing this little volume, to state, that it is strictly conchological, and that it is compiled for the use not only of those who wish to acquire an elementary acquaintance with the subject, but also of authors and others, who, desirous of extending their knowledge and pursuing their researches, require a book of reference, containing a general outline of what has been done by those who have trodden the same path before them. It has been thought advisable, for general convenience, to arrange the principal part of the information in alphabetical order: adding tables of the systems of Lamarck and De Blainville, to facilitate the systematic pursuit of the science. Persons of the class first alluded to, will find great assistance in the explanation of technical words, their application being further illustrated, in most cases, by a reference to the figures; and, although they might have been multiplied, it is trusted that enough are given for every useful purpose. The definition of the Classes, Orders, Families, and Genera, in the system of De Blainville, and a tabular view, are presented for the use of those who prefer it, or who wish to compare it with that of Lamarck. In the explanation of the figures, will be found a systematic arrangement of shells, according to Lamarck, including the names of genera established or proposed since the publication of his system. The descriptions of established genera have been rendered as concise and clear as possible. It is hoped that no essential characters are omitted, and that those living authors, whose proposed generic distinctions have been passed over in a few words, will not have to complain of want of justice in the attempt to interpret their meaning. In most cases the generic name will be found accompanied by its derivation. This has been done, in the hope of assisting the memory by associating the meaning of a term with some peculiarity in the thing described. At the end of each description of a genus, some general observations occur, pointing out the principal character which distinguishes it from others, to which it is nearly allied; and also stating the geographical or geological distribution and habits of the animal. The above descriptions and definitions are illustrated by a series of plates, containing above 500 etchings of nearly as many proposed or established genera, arranged in Lamarckian order, so as to show at a glance all the generic forms of each family. And, although from their number, they could not be very highly finished, it is hoped that they will be found characteristic. The compiler cannot replace his pen without acknowledging, with filial gratitude, the kind assistance of one who has sacrificed much of his time in bringing his knowledge and experience to bear upon the correctness and utility of this humble attempt to remove some of the difficulties to which the commencement of this, as well as of every other study, is exposed. PREFACE TO THE SECOND EDITION The favourable reception and rapid sale of the first edition of the Conchological Manual having rendered a second necessary, the Author takes this opportunity of explaining the nature of the alterations which have been made. In doing this, he has to thank his friends for their suggestions, which, together with his own increased knowledge and experience, enable him to present a more complete and satisfactory work to the scientific public. For the further convenience of those who are studying the rudiments of the science, an entirely new Introduction is given, in which, commencing with the structure and gradual developement of the shell, the author has endeavoured to explain the general principles of Conchology in systematical order. This Introduction is illustrated by 100 wood-cuts, which will be found greatly to assist the Student. The definitions have been rendered more full and complete than before, and the Author has profited by some manuscript notes communicated by a scientific friend, to whom he desires to present his humble acknowledgments. Upwards of four hundred explanations have been given of words which did not appear in the former edition, three-fourths of which are of generic and subgeneric names. A large number of notes, referring to the geographical distribution of the genera, have been added from the pen of Mr. G. B. Sowerby, Senior. The plates have been carefully improved; and three, containing upwards of eighty figures, have been added. On the whole, it will be found that the amount of matter has been nearly doubled; all the defects, as far as they have been discovered, have been removed, and every means used of making the present edition as useful as possible. NAMES OF AUTHORS ABBREVIATED.
INTRODUCTION. The Science of Conchology affords a very delightful and instructive amusement for the leisure hours of those who, retiring occasionally from the gaieties of fashionable life, seek pleasure in the quiet contemplation of some of the smaller, but not less wonderful operations of creative wisdom. And, although the study of shells would be more complete, and rank higher in the scale of philosophical pursuits, were it always accompanied by that of the animal inhabiting them, it nevertheless presents means of intellectual gratification, to many who cannot follow it beyond the cabinet and the boudoir. These may examine with admiration and mental improvement, the beautiful colouring and architecture of these wonders of the deep, they may exercise their taste and judgment in the selection and arrangement of specimens, and their discrimination in detecting and appreciating the distinctions upon which the arrangement is founded. It is but little that can be known of the subject without forming a collection of greater or less extent; for, as it would be uninstructive merely to delight the eye with the bright colours and elegant form of shells, without possessing correct information respecting them, so it would be insipid and useless to learn technicalities without being acquainted by personal observation with the subjects to which they are applied. The first endeavour should, therefore, be to obtain a few shells as examples of the larger divisions, and, when these are understood, to proceed with the smaller groups, until a collection be formed to represent as many generic forms as possible. It may be as well here to advise those who are forming a collection to be very particular in every practicable instance to have the shells properly named at the time of purchasing; as it will save much trouble, and materially assist in the attainment of the desired object. To this end, recourse should be had to those naturalist tradesmen, who unite the attainment and diffusion of real scientific knowledge with their commercial pursuits. Supposing, however, that the person who desires to learn the science, possesses a small parcel of unarranged and unnamed shells, without any previous acquaintance with the subject, the following introductory explanations, are drawn up with the view of enabling him, without further assistance, to obtain a general insight into its principles, equal to that of those who have studied it long and laboriously. To effect this, he must read them, carefully comparing the descriptions with the figures referred to, and with the specimens which he may have at command. After describing the nature of the science and defining its objects, we shall proceed to explain the structure of those objects, and the manner of their growth. We shall then enter somewhat minutely into the principles of classification, the distinctions upon which they are founded, and some of the technical terms used to express them. After which we shall pass through the arrangement of Lamarck, defining the general divisions adopted under the terms of "Classes, Orders, and Families," as far as they are capable of definition. The subdivision of the latter into genera will only be entered into so far as to enumerate the principal of them, the more minute descriptions being reserved for the alphabetical part of the work. Let none be discouraged by the number of generic distinctions proposed and adopted in modern times; for if well defined, they will be found to facilitate rather than encumber the science. The knowledge of species must be the foundation of every system, and the greater their number, the more necessary it becomes to subdivide them; if, for instance, all the species now known were to have been included in the 50 genera of LinnÆus, a single genus would have contained many hundreds of incongruous species, in which case it would be much more difficult to remember them, than if they were to be divided into a far greater number of genera. Every well marked division, however arbitrary its limits, tends to simplify the subject, and to facilitate the researches of the student. NATURE OF THE SCIENCE. Conchology is the study of shells, viewed and described as to what they are either in themselves, or in relation to the soft, inarticulate animals which produce them, and of which they form a part. These animals are called Mollusca, and perhaps the best general description of them will be found in De Blainville's "Manuel de Malacologie et de Conchyliologie." The following is a translation, "Animal in pairs, the body and its appendages soft, inarticulate (not jointed), enveloped in a muscular skin, commonly called the mantle, which is extremely variable in form, and has developed either within or upon it a calcareous portion, consisting of one or several pieces, commonly called a SHELL." The term Mollusca was formerly restricted to those soft animals which were destitute of shells, although possessing in other particulars, the characters described above, and it was used in order to distinguish them from the Testacea, which were covered or internally supported by calcareous parts. In the system of LinnÆus, the soft portions are first arranged under the general designation of "Vermes Mollusca," and described without regard to the presence, absence, or character of the shells; and then the shells are separately characterized under the appellation of "Vermes Testacea," without any further notice of the animal, than an indication of the genus to which it belongs; thus the animal of CyprÆa is said to be a Limax, and that of Tellina a Tethys. The nearest approach to correctness, and the most philosophical method of study will be found in the modern system, adopted by Lamarck and his followers, of observing these animals as a whole, and arranging them according to the assemblage of characters which they present; of course taking into consideration the existence or non-existence, form and structure of the shell, on the same principle, which, in arranging the vertebrated animals would lead us to study the hair, hoof, nails, claws, &c. as well as the other parts. At the same time, it must be admitted that there are many private collectors of Shells who would find it a difficult, if not impossible task to study minutely and successfully the soft parts of the Mollusca. Ladies, for instance, could not be expected to handle with pleasure and perseverance, these fleshy substances, which in order to be preserved from putrefaction, must be kept in spirits; and yet such persons may, with improvement and advantage to their own minds, enjoy the interesting and scientific amusement of studying and arranging the clean and beautiful natural objects which are so easily preserved, and so exquisitely curious in their structure. Let it also be remembered, that if shells had not been rendered commercially valuable, by the zeal and emulation manifested by mere Conchologists for the possession of rare specimens, few travelling merchants and sea captains would have thought them worthy of a corner in their cabins. In this case, few specimens being brought to the country, the more Philosophical Naturalist would have been left without the means of obtaining materials to work upon, or of attracting public attention to his favourite pursuit. On account of these and other considerations, it has been thought advisable that the present undertaking should bear a purely conchological character. The peculiarities of the shells alone being detailed for the assistance of those who collect and study them, while at the same time, in deciding upon their affinities and places, in the arrangement, it will be necessary to take advantage of the conclusion to which those have arrived, who have studied the animal in all its parts. And the conviction must be expressed, that if ever a complete Natural System shall be formed it will result from the labours of the last mentioned class of naturalists. DEFINITION OF A SHELL. Before entering minutely into the description of shells, it will be necessary to distinguish from the true testaceous Mollusca two kinds of animals which have formerly been associated with them. Of these, the first is the class of Crustacea, consisting of crabs, crayfish, &c. These differ from shell-fish, not only in structure and chemical composition, but also in the fact that the animal has jointed limbs, and that the substance of the flesh is inseparable from the hard external covering, which invests each particular joint as with a sheath; whereas the Molluscous animal is but partially attached to its shell, from which it possesses the power of partly withdrawing and returning. The second class is that to which the sea-urchin, or Echinus, belongs, of which there are many genera and species. The testaceous covering of Echini is composed of a number of small pieces, placed edge to edge, forming a more or less globular external covering to the flesh, which is supported in the centre by a number of bones leaning upon each other in a pyramidal form. The test is of a fibrous texture, guarded on the outside with moveable spines, which turn on ball and socket joints. A true shell is composed of one or more calcareous pieces, commonly called valves, each piece formed by a series of layers, applied obliquely upon each other, in such a manner that each new layer begins within, and terminates a little in advance of the one before it. STRUCTURE AND GROWTH. We shall now endeavour to describe the manner in which the growth of each separate valve, or each regularly formed shell, proceeds from the nucleus. Before the young animal has left the egg, if it be an oviparous species, or the body of the parent if viviparous, the nucleus of the shell is generally formed, and specimens are sometimes preserved in which the young shell is seen within the egg, as in the cut, fig. 1, 2; or adhering to the inner surface of the full-grown shell by the dried mucus of the animal, as seen in fig. 3. Figs. 1-3. 1. Egg of a Bulinus. 2. The same broken, shewing the young shell. 3. The young of a Paludina, as seen in the aperture of the shell. In both cases, the nucleus is generally of a more horny and transparent composition than the parts subsequently produced. As soon as the animal is hatched, or, in other words, leaves the egg or body of the parent, of course it begins to increase in size, and to require a corresponding enlargement in the shell. To effect this, a small quantity of mucus substance, secreted by the mantle of the animal, is deposited on the edge of the aperture. When this is dry and become sufficiently hard, it is lined by a more calcareous secretion; and these together form a new layer, which is followed by others in succession; each new layer being larger than the one that preceded it until the whole being complete, the full-grown animal is invested with a shell commensurate with its own proportions. Thus from the apex or nucleus the formation proceeds, as it were, downwards, taking the shape of the part which secretes it, on which it is in a manner moulded. The nucleus, or first formed portion, may for technical purposes be considered, mathematically, as the apex of a spiral cone. And here it must be observed, that whether the shell consist of one or several pieces, each piece has a separate nucleus, and the process of formation is separately repeated with each. The word cone is used for convenience, and its meaning extended so as to include all those structures which commencing at a point enlarge downwards. Fig. 4. 4. Imaginary cone. a. Apex. b. Base. l. Lines of growth. From the apex, the next layer is deposited on its edge, and advancing beyond it necessarily adds to its extent. Thus, suppose for the sake of illustration, the part marked a in the diagram, fig. 4, to represent a nucleus, the cross lines (l) will shew the consecutive layers, which enlarge their circle as they add to their numbers. This disposition of shelly matter into layers is marked externally by concentric striÆ, or lines of growth, while on the inside the edges of the laminÆ are consolidated into a kind of enamel. If a perpendicular section of a solid portion of a shell were magnified, it would present, in many instances, an appearance resembling the diagram, fig. 5; a may be taken to represent the horny part of the layers which form the outer coating, named "Periostaca," or "Epidermis;" the undulating line b, is formed by the edges of the calcareous layers, and causes the striÆ, or lines of growth, which are often distinguishable on the surface of the shell; the space c is the middle part of those layers, and at d they are consolidated into the enamel which lines the interior. Fig. 5. 5. Supposed section of a part of a solid shell. In some species the layers are irregularly grouped together, and their edges overlap each other, so that they are easily separable, and advancing beyond each other, give a leafy appearance to the external surface. This structure is termed foliaceous. A very familiar instance of this may be observed in the common oyster. If a specimen of this shell be broken, the substance will be seen to exhibit a degree of looseness, and a magnifying glass will enable the student to trace distinctly the laminÆ of which it is composed. The accompanying representation of a magnified section (fig. 6) will shew at a, the external surface, with the foliations or leaves; at b, the parcels of layers which form them; and at c, the pearly structure produced by their consolidation, and by the subsequently deposited enamel which covers their external surface. Fig. 6. 6. Section of an oyster shell enlarged. CLASSIFICATION. The classification of shells, that is, their systematic arrangement into classes, orders, families, genera and species, cannot be made to depend entirely upon the characters observable in them, viewed by themselves; for this reason, that many similarly formed shells form the habitations of animals perfectly distinct, and that many molluscous animals are found to agree with each other in every respect but in the form of their testaceous support. There are, however, many important distinctions to be observed in the shells themselves, leading to the establishment of many of those very divisions, which would afterwards be confirmed by an examination of the soft parts. It is necessary to attend, as far as means and opportunity will allow, to all the points of difference, both in the shell and in the animal, in order to form, and in some instances even to appreciate, a generic or larger distinction. It will therefore be our endeavour to explain the general principles upon which those distinctions are formed, and the manner in which they are applied and expressed in detail by scientific writers. NUMBER OF PIECES, OR INDEPENDENTLY FORMED PARTS. The first, most simple and obvious division of shells, is that which results from the number of separate pieces composing them. Hence the distinction implied by the terms UNIVALVE, or consisting of a single piece; BIVALVE, or composed of two pieces; and MULTIVALVE, or composed of more than two. For an example of univalve, take a common whelk; for a bivalve, take a muscle or a scallop; and for a multivalve, the barnacle, or balanus, found adhering to the common oyster. But although this arrangement may appear at first sight perfectly easy and plain, some explanation will be necessary in order to guard the student against understanding the above expressions in their strictest sense, without qualification. Thus the univalves are said to consist of a single piece, or spiral cone; but it would be more correct to speak of this piece as forming either the whole or the principal part of the shell: for there is in many instances, a much smaller flattened piece attached to the foot of the animal, which being drawn in when it retires, closes the aperture as with a kind of door, to which in fact the word valve might be very properly applied; it is called however the OPERCULUM, and the little horny plate, frequently drawn out by means of a pin from the aperture of a periwinkle, will present a familiar example. Fig. 7. Accessary valves of a Pholas. The same may be said respecting the bivalves; for besides the principal portions or valves of which the shell is composed, there are in many species, one or two smaller separate portions, named "accessary plates" by some authors. They are fixed by means of cartilages, on the back of the hinge.—The engraving, fig. 7, represents the accessary valves of a species of Pholas, which was on this account arranged by LinnÆus with the Multivalves. Nearly allied to the Pholades is a set of shells to which De Blainville has given the name "TubicolÆ," or inhabitants of tubes. In this case, the bivalve shell is connected with a testaceous tube or pipe, to which it is attached either by one or by both valves, or in which it lies attached only by the cartilages of the animal. In the genus Aspergillum, the two small valves are soldered into the sides of the tube in such a manner as to constitute a part of it. One of these shells, called the Water-spout, might be taken up by a person not aware of its real nature, and regarded as a pipe or tube prettily fringed, and nothing more; but upon a closer examination, he would find the two valves, the points of which are visible from the outside of the tube. HABITS—Land, Fresh-water, or Marine Shells. Another distinction, leading to important results in classification, is that which is derived from the nature of the element breathed by the Mollusc. And although this consideration belongs more especially to the study of the animal itself, yet the habits of the animal materially influence the structure of the shell. The Terrestrial or Land Molluscs live on land, breathe air, and feed on plants and trees.—Those who find pleasure in horticultural pursuits will at once call to mind a too familiar example of these Molluscs in the common garden snail. The Land-shells are all univalves, and constitute a family in the Lamarckian system under the name "Colimacea," or snails, corresponding with the Linnean genus Helix.—They are generally light in structure and simple in form. The Aquatic, or Fresh-water Molluscs, such as the Planorbis, commonly called the Fresh-water Snail; the Unio—known by the name of Fresh-water Muscle, is found in ponds, ditches and rivers. The epidermis of these is generally of a thick, close-grained character, and they are subject to corrosion near the umbones. There are but few genera of fresh-water shells besides the Uniones, among bivalves, and the "Melaniana" among univalves. Concerning the former it may be observed, that they are all pearly within, and the colour of the thick horny coating embraces all the varieties of brownish and yellowish green. The Marine, or sea-shells, belong to all the classes and orders, and include by far the greater number of species. They vary in the habits of the animal, and consequently in the situations in which they are found. Some are found buried in sand and marine mud, and are named "ArenicolÆ" or inhabitants of sand; others in holes of rocks and other hard substances, then they are named "PetricolÆ,"—some of these latter form the holes in which they live by corroding or eating away the stone. A section of these form the family of "LithophagidÆ," or stone-eaters, of Lamarck. Others, again, take up their parasitical abode in the bodies of animals, and feed upon their substance; as for instance, the Stylifer, which is found in the vital part of star-fish, and Coronula, and Tubicinella, found buried in the skin of the whale. LOCOMOTION—Attached, Unattached. A much more subordinate source of distinction arises from the freedom or attachment of the shells. Some of them float or walk freely in their natural element; others are fixed or attached to foreign bodies. Among those which are attached, there is again a difference as to the mode of attachment. Some are united to foreign bodies by means of a glutinating substance, secreted by the animal, and joining part of the surface of the shell to that of the stone, coral, or other substance. In this way shells are fixed to each other in groups; this is the case with the Spondyli among bivalves, and the SerpulÆ among univalves. M. de Blainville applies the term "FixÆ" to these shells. Others are kept in a particular place by means of a Byssus or Tendinous fibrous line or bunch of silky hairs, acting as a cable, and allowing the Mollusc to ride as it were at anchor. This Tendon is connected with some part of the animal from which it passes through an opening or hiatus in the shell, as in the Terebratula and the Mytilus. In the former, represented by the cut, fig. 8, the tendon passes through a perforation in the upper valve; and in the latter, Mytilus, fig. 9, the byssus passes out between the valves. Before proceeding to explain the characters of the different groups, according to the modern system of classification, it may be desirable to explain the terms by which the different parts and characters are described, and to shew the manner in which the shells are measured. For this purpose we shall treat of the general divisions separately. We begin with UNIVALVE SHELLS. In considering Univalves merely with reference to their mathematical construction, the first point demanding our attention is, whether they are symmetrical or non-symmetrical, or, in other words, whether a straight line drawn through the shell would divide it into two equal parts. The greater part of univalves are non-symmetrical, being rolled obliquely on the axis; but many are symmetrical, being rolled horizontally on the axis. The Nautilus presents an illustration of the latter; the Snail is a familiar example of the former. Symmetrical Univalves. In describing these it will be well to commence with the most simple form, such as the Patella,—taking a conical species as an example. In this it will be observed that there is no winding or curvature, but a simple depressed cone, and that the line a, p, divides it into two equal parts. The anterior, a, (cut, fig. 10) is known by the interruption of the muscular impression which surrounds the central disc (d.) This interruption of the muscular impression is in the place where the head of the animal lies in the shell. The impression itself is caused by the fibrous muscle which attaches the animal to the shell. The apex (a) in Patella, generally leans towards the anterior (a) part of the shell, and away from the posterior (p); and this circumstance has caused some mistakes, because in Emarginula the apex leans towards the posterior; and students, instead of examining the muscular impression, which is the only criterion, have only noticed the direction in which the apex turned, and concluded that to be the anterior, towards which it inclined. The lines or ribs running from the base to the apex of the shell, in the direction r, are called radiating lines; and those which encircle the cone in the direction c c, from front to back, are very properly described as concentric. The length is measured from front to back in the line e; the breadth, from side to side, in the line b; and the depth from the apex to the base. Let it be observed that patelliform, or limpet-shaped shells are not all symmetrical; Umbrella, Siphonaria, Ancylus, &c. will form exceptions, of which we have yet to speak. And the learner may also be reminded that the Limpets themselves are not all regular in their form: for as they adhere to rocks and other rough surfaces, and are so little locomotive, in many instances they partake of the inequalities of the surface, and conform to its irregularities. This adherence is not effected by any agglutinating power in the animal, nor by any tendinous process like that described above; but simply by means of the foot of the animal acting as a sucker. The next variation in symmetrical univalves is to be observed in the tubular, curved form, the example of which will be the Dentalium, fig. 12. Fig. 12. Dentalium Elephantinum. This has an opening at the anterior termination a, called the aperture. The opening at the posterior end (p) is named a fissure, or perforation. The ribs running along the sides of the shell are longitudinal, or radiating. And the lines round the circumference are lines of growth, or concentric—each one having in succession, at earlier stages of growth, formed the aperture. They are described as concentric, or transverse. Symmetrical Convolute Univalves. The Nautilus, the Spirula, the Scaphite, and the Ammonite are the leading types of this form; but when we use the term symmetrical, in reference to these, the word must not be understood in its strictest sense, for no shell is perfectly symmetrical: but it means that there is no perceptible difference in the proportion of the two sides; as in the human body, the right side is larger and more powerful than the left, yet to a degree so small that it gives no apparent bias to the figure. CHAMBERED SHELLS. Many of the shells now under consideration are chambered, that is, the internal cavity is divided into separate compartments by plates reaching across it, named Septa; and the only connection between the chambers is formed by the small pipes passing through them, to which the name of Siphon is attached. Septa. The septa are simple in some species, as in the Nautilus, fig. 13. In others they are undulated, having waved edges, as in some species of Ammonites; in others they are angulated, as in Goniatites, fig. 480 in the plates; and in the greater number of instances, among the Ammonites, they are arborescent, or branched. Figs. 13-15. 13. Section of Nautilus.—14. Undulating Septa.—15. Arborescent Septa. In the above section of a Nautilus, fig. 13, diminished in size, showing the whorls and chambers (c), it will be seen that the edges of the septa (s) are formed in one simple curve. In fig. 14, the upper part of an Ammonite, the undulating line will be seen; and in fig. 15 a specimen is given of the arborescent septa. Siphon. The Siphon is dorsal when placed near the outside of the whorls; central when near the middle; and ventral when near the inside of the whorl, or that part which leans against the last volution. When it passes uninterruptedly from one chamber to another, it is described as continuous, as in the case of Spirula; when, on the other hand, it only passes through the septum a little distance, and opens into the chamber, as in Nautilus, it is discontinuous. Whorls of Symmetrical Univalves. They are disunited when they do not touch each other, as in the case of Spirula (fig. 471 in the plates); but in the contrary case they are said to be contiguous. In some species of Nautilus the whorls overwrap each other in such a manner that the early whorls are entirely covered by the last, the edges of which reach to the centre of the disk: the spire is then said to be hidden; as in the Nautilus Pompilius. In Nautilus umbilicatus the spire is nearly hidden, the whorls not quite covering each other; but in the greater number of the Ammonites, the largest part of the preceding whorls is seen. To express the degree in which the whorls overwrap each other, has caused much difficulty in concise descriptions. Perhaps it would be well to apply the term spiral disc to so much of the shell as is seen besides the last whorl, and to describe it as large or small in diameter, compared with the whole: or to say that the whorls of the spire are half, or one-third, or one-fourth covered, as the case may be. Aperture of Symmetrical Univalves. In Ammonites Blagdeni and some others the aperture is of an oblong square; it is then said to be sub-quadrated; in Nautilus triangularis it is angulated; in Ammonites Greenoughi it is of an interrupted oval shape, described as elliptical. In the greater number of Orthocerata, it is rounded or circular. The entrance of the last whorl into the aperture of some rounded species of Nautilus causes it to take a semi-lunar form; if rounded at the sides it is said to be reniform or kidney-shaped; if pointed at the sides it is semi-lunar; and in some species of Ammonites, it is five-sided or quinque-lateral. Measurement of Symmetrical Conical Univalves. The width is measured across the aperture, which is the widest part of the shell. The length (l) from the dorsal part (d) of the aperture to the dorsal part of the whorl (d) on the opposite part of the shell. The ventral part of the whorls is that nearest to the axis, and the dorsal that which forms the outline of the figure. NON-SYMMETRICAL UNIVALVES. These are conical, irregular, spiral, or convolute. The conical form is when there is no enrolment of the apex. Although the PatellÆ were described as symmetrical, there are several species of Patelliform shells which are not symmetrical. In Umbrella, for instance, the apex is oblique, the shells being placed obliquely on the animal. In the genus Siphonaria, there is a groove on one side, where the brachia or gills of the animal rest. In the genus Ancylus, it will be observed that the apex bends on one side, and the animal is like the LimnÆa, which has a spiral shell. The cup and saucer Limpets, or CalyptrÆdÆ, present a group which requires to be described, differently from the symmetrical or true Limpets. Their structure is very curious, and they vary considerably among themselves, some of them being simply conical, others nearly flat, or discoidal, and others more or less spiral. But their principal peculiarity consists in their having a small internal process or plate variously shaped, commonly named their septum. Septa of Limpets. The septa of Limpets assume a variety of forms, the principal of which will be seen in the accompanying engravings. The form from which the group derives its generic appellation is that of the cup-shaped or Cyathiform species (fig. 17). In the CrepidulÆ, or Slipper-Limpets, the septum is flat, reaching across the opening, like the deck of a vessel; it is then described as transverse (fig. 20). In CalyptrÆa Equestris, it has two prominent points, and is described as bi-furcated (fig. 18). In another species, it is a three-sided plate rather spiral at the apex (fig. 19). Measurement of Cup and Saucer Limpets. The line marked a, p, ll indicates the direction in which the shell is to be measured for length. a indicates the anterior, p the posterior. The line d (fig. 23), from the apex to the base, is the depth. The line b (fig. 28), is in the direction of the breadth. Irregular non-symmetrical Univalves. Serpuliform shells are irregularly twisted (tortuous) hollow tubes, which were formerly considered to have been secreted by a kind of worm, but now known to be the shells of true Molluscs, of a kind not very widely differing from those which have regularly spiral shells. The greater part of these are attached to foreign bodies, or to each other in groups. Some are attached by the whole length of the shell, they are then said to be decumbent. Some of these are coiled round like the Spirorbis, the little white shell seen on the carapace of the Lobster or on leaves of sea-weeds; they are then said to be discoidal; others again, such as the Vermetus, approach more nearly to the spiral form. The deviation from the regular spire only taking place after the few first volutions. SPIRAL NON-SYMMETRICAL UNIVALVES. As these constitute the largest class, it will be necessary to dwell upon them in detail. First as to measurement. The length is measured from the apex, to that part of the aperture a (fig. 24), at the greatest distance from it. The breadth is in the opposite direction. The anterior, or front part of the aperture, is marked a, where the head of the animal protrudes. Spire of non-symmetrical Univalves. Figs. 25-32. Fig. 25, obtuse; 26, acute; 27, 28, decollated; 29, concave; 30, papillary; 31, mammellated; 32, discoidal. In counting the whorls of which the spire consists, we commence at the apex, and reckon downwards to the last, or body whorl. The spire is described as being long or short in relation to the aperture: in which case, all that is above the aperture is measured with the spire. Its apex requires particular notice, as the character of the whole shell frequently depends upon the particulars observable in this part. It is sometimes obtuse, or blunt; sometimes acute, or sharp. In the Cones it is frequently flat, and in Planorbis it is concave. It is sometimes of a different structure from the rest of the shell, retaining the horny and transparent appearance which characterized it when the animal was first hatched. The Tritons present an instance of this, although it is not always observable, owing to the tenderness of the substances which causes it to break or fall away in many specimens. A very remarkable instance also occurs in Bulinus decollatus (cut, fig. 27, 28), so named, because the apex, to the depth of several whorls, falls off, and the shell is decollated. In this, and many more instances, among PupÆform land shells, the occurrence of this circumstance seems to be by no means rare or accidental, a provision having been made for filling up the opening by a septum. A papillary apex is one which is swelled at the extremity into a little rounded nob, or nipple; and a mammellated apex is one which is rounded out more fully into the shape of a teat. Whorls. The spire is described as consisting of numerous or few whorls, and sometimes the number of them is particularly stated. A whorl consists of one turn of the spiral cone. The whorls are described as flattened, when the sides are not bulged out so as to cause the outline of the spire to deviate considerably from straightness: when the contrary is the case, the whorls are said to be ventricose, and either rounded or angulated. The degree of rapidity with which the whorls become enlarged presents an important source of distinction. The suture, or seam, which separates one whorl from another is also noticed as being distinct or otherwise; canaliculated, or grooved; or covered by an enamel, which in some instances is swelled into a ridge or tumid. |
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