regarding as opercula, but the structure of which has not previously been examined in detail, I am inclined to add two other bodies belonging to ctenobranchous molluscous animals, which have hitherto been generally regarded as anomalous. The first of these is the support, as it was called by its discoverer M. de France, or under valve, as it has since been regarded by some English conchologists, of the genera Capulus and Hipponyx. I am induced to regard this body as analogous to the opercula of other spiral shells, because, on an attentive examination of the animals, I find that it is attached in the same situation and not on the under side of the foot, as most conchologists have supposed; the foot being folded on itself, and the walking disk of other Gasteropods being in these animals (which never move from the place of their first attachment, and consequently require no such expansion), represented by a few crumpled folds placed between the part to which the shelly plate is attached and the head. In this idea I am further confirmed by a somewhat similar structure of the foot in the genus Vermetus, where the back of that organ represents a truncated cylinder, filling up and closing the mouth of the tubular shell. This foot is crowned by a horny operculum, and the walking disk is reduced to a narrow flat band, passing along the front of the cylinder, which band is in some species terminated by two conical processes, situated between that part and the base of the head the processes have been described as tentacula, which they resemble in form. The shelly plate or operculum of Capulus is formed of concentric shelly laminæ, with a nearly central nucleus, and differs from all other opercula at present known, in being immediately attached, by its outer surface, to other marine bodies, like the lower valves of the Oyster and of Crania, and thus forming the medium by which the animal is retained in its place. The mouth of the shell being nearly as large as the cavity, the adductor muscle, as in other shells of this form, is divided into two broad bands, forming a horse-shoe-shaped, posterior, submarginal, muscular scar, and the operculum is marked with a similar impression.

The second body to which I refer is the vesicular appendage, placed on the back of the hinder part of the foot of the animals belonging to the genus Ianthina,* which appears to

:

* Cuvier at the time of publishing his Anatomy of Mollusca, appears to have entertained the same theory, for he there properly describes this body as attached to the hinder part of the foot, a little below the usual place of the operculum; but in his Animal Kingdom he seems to have abandoned it, and describes the animal as having no operculum, but having a vesicular organ under its foot.— Règne Animal, ed. 2, tom. iii. p. 84.

assist in floating the animal on the surface of the sea, and probably also in supporting the eggs after the death of the parent. This float, as it has been called, I am inclined, from its being situated in the same position as other opercula, to regard as analogous to those bodies in the neighbouring genera.

In the "Medical Repository" for 1821, I first called the attention of conchologists to the importance of the characters furnished by the operculum for the distinction of genera and families; and this subject, although almost neglected in this country, has since been pursued with great assiduity by M. de Blainville and other French conchologists. I have fortunately had an opportunity of examining, either in the cabinet of the British Museum or in the Continental collections which I have visited, the animals of the greater number of genera of shells, and have been thereby enabled to determine that the form and structure of their opercula offer some of the most constant characters for the distinction and arrangement of families and genera; while, on the other hand, I have convinced myself that systematists have been in the habit of placing too much reliance on the mere fact of their presence or absence as a family character, inasmuch as that circumstance alone will scarcely prove of generic importance. Thus in the genus Voluta, the animals of the eight or nine species which I have examined are all destitute of opercula, except Voluta musica, which has an operculum of moderate size. The Olives and Mitres are in the same predicament, most of the large species being destitute of opercula, while the smaller species of both genera are furnished with rather large ones, as may be easily seen in specimens of Oliva eburnea, O. zonalis, or Mitra striatula, in which the animals have been dried; and shells in this state are not uncommon in collections. The species of Cones offer in this respect the same variations. These observations will explain the apparent contradictions of describers, and the frequent controversies that have taken place as to whether these and some other genera have or have not opercula. That their presence or absence is not a family character may be inferred from all the genera of Buccinidæ being provided with them except Harpa and Dolium. And this leads me to remark, that many genera and species which have very large mouths, in comparison with others of the family to which they belong, are destitute of, or have very small, opercula, whilst the others have moderately sized or even large ones. Thus the wide-mouthed Cones, as, for example, Conus geographicus, have no operculum, whilst the other species

have a distinct one: and the genera Cryptostoma and Concholepas have very small opercula, in comparison with the size of their mouths, whilst the other genera allied to them have their opercula nearly as large as the mouths of their shells. The genus Vermetus is in this respect very remarkable most of the species have the operculum as large as the mouth of the shell; but there is one in the British Museum which has an operculum very small in comparison to the size of the body of the animal, and not one fourth part of the diameter of the tube of shell. Some species of this genus, indeed, are described as having no operculum; and the observation of the above fact induces me to give credit to the description, which I was at first inclined to doubt.

But of all the variations in this particular, those of Capulus and Hipponyx are the most remarkable: some species appear always to have an operculum, which, like the under valve of Crania, differs in thickness according to the form and degree of exposure of the substance to which it is attached-others, as the common Capulus hungaricus, are generally without operculum, although, according to the observations of Dr. Turton, the last-named species sometimes forms a thin support;-and there are others which, instead of forming an operculum, make for themselves (as has been already alluded to in this paper), a cavity in the substance of the shell to which they are affixed, which is marked with a lunate ridge, corresponding with the muscular scar of the operculum, and doubtless occasioned by the attachment of the adductor muscles to that part of the shell, which is thus protected from the dissolving power of the mantle.

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LETTER XXIV.

THE CONCHOLOGIST'S NOMENCLATURE.

MOLLUSCA.

THE ACEPHALOUS

WITHOUT a collection of shells you will never become a good conchologist. The collection need not be extensive nor rich in rarities, but it should include specimens illustrative of the families and principal genera; and, that it may fulfil its object, these specimens must be named by your own study. The aid of figures ought at first to be rarely and reluctantly required. The adage that the eye is a readier teacher than the ear, is only partially applicable to objects of science, for no figure can represent the object in all its aspects. If by the aid of the artist we gain a certain amount of knowledge readily, we as certainly lose the benefit resulting from the habit of correct analysis, which the comparison of the object in hand with the description is so well calculated to teach. I have noticed that the student who ever resorts to figures, and depends upon them for the identification of his specimens, commits not only many errors, but soon becomes incapable of enduring the patient criticism which is necessary to ascertain the station and name of nearly allied or abnormal species; and his study, giving no aid to mental culture, loses much of its attraction. Hence, such a student either soon forsakes the pursuit,-it is too facile for continuous interest, - or he lands in mere amateurship. To make your choice pleasant and enduring, you must go deeper and master principles and details, and give an importance to the least of them. You must examine for yourself, and learn the luxury of solving difficulties. Do not, therefore, be seduced by the prevalent use of "Illustrations" to become an iconologist; but let your vade-mecums be the descriptive works of such authors as Linnæus, Otho Fabricius, Müller, and Montagu.

To understand the descriptions of these masters of our science, you must, of necessity, learn the peculiar language in which they are written, - the meaning of the words they have invented or applied, to designate the exterior organs of the animals in question, and their varied configuration and sculpture. In conversation, you may hear people complain

of our nomenclature, as a medley of barbarous and jawbreaking words; but were these as harsh and unspeakable as were the surnames of the Scotch to the musical ear of Milton, yet the fact would not do away with the necessity of your learning them. The artisan would smile at the simplicity of the man who should require him to explain the parts of the intricate machinery he guides, without the use of the terms of his craft. It is needless, however, to dwell upon such an obvious point; and so, with the humility of an apt scholar, you will now follow me through some dry details, into which it may be impossible, perhaps, to enter without the determination to turn them into future profit.

Fig. 82.

I. TERMINOLOGY OF THE TUNICATA.

The terms used in describing the constituents of this class are few. The Tunicata are- simple (Fig. 82), when every individual is complete in itself; social, when several individuals are connected together by a creeping tube; and compound, when many are organically combined and associated together to form a

b

common mass.

The external covering of the single individual, and of the compound mass, is called its mantle or tunic; and the inner coat which immediately invests the viscera is the branchial sac. There are two corresponding apertures in the mantle and sac; one, distinguished in general by being more elevated than the other, is the branchial (Fig. 82, a), and the other (b) is the anal orifice or vent. The

"It is frequent, even with some who pretend to be naturalists, to vilify the fundamental parts of natural history; who view the particular species and bodies in nature; their systematic arrangement; their correct denomination; and the description of their parts and properties; as a study too minute, frivolous, and beneath their notice; whose large views are only directed to what they call the volume of nature, and the great lines in natural history. But I know of no great lines in natural history that are not composed of small ones; nor have I ever had occasion to admire any man's knowledge concerning a great line, that was ignorant of its component parts.

"As for their volume of nature-like other volumes, it consists of pages; and those pages, of lines, words, and letters. But without an acquaintance with these, we have no more right to pretend to understand this boasted volume, than we would to have to understand a book, whose letters, words, lines, and pages, we have never perused."-DR. WALKER, Essays on Nat. History, p. 334.