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The shells called univalve, or those composed of one part only, are far more numerous than the two preceding, both in genera and species; and it requires a considerable degree of attention to discriminate many of the species, as they run into each other so much; and they are divested of the strong and distinct character afforded by the teeth of bivalves; besides many of the species there are several varieties.

In the examination of shells of this order, the general contour or outline of the whole shell is the first particular to be attended to, as this leads to those distinctions necessary in the definition of simple, spiral, or turbinated shells, Univalves with a regular spire, and those without a regular spire. The genera of this order are formed principally from the shape of the aperture, taken in conjunction with the general shape of the shell; from the spire being lengthened or depressed, being with or without a canal, the length of the beak and its direction, together with the particular form of the outer lip: the colour of shells only serves as a specific distinction, and cannot in this respect, in all cases, be depended upon, although in others it is an unvarying test. The particular manner in which the spots are disposed frequently characterises species.

Apex. The summit, tip, or highest part of the spire. Plate II. figs. 7, 8, 9, A A A.

Base is the opposite extremity from the apex, or tip of the spire. In shells with a beak it implies the tip of such beak; Plate II. fig. 11, B. In shells without a beak it is understood to be the lower part, as before-mentioned, opposite the apex; Plate II. figs. 8 and 9, B B. In the Patella and some others, the base of the shell is that part on which it rests when it is laid on its mouth. In the Dentalium and Teredo it is the wider end.

Body of the shell is the first or lower whorl of the spire, in which the aperture is situated, and is in general longer than the remaining whorls. Plate II. figs. 10 and 12, F F; and Plate III. figs. 7 and 9, F F.

Front of the shell is that side where the aperture is situated. Plate II. fig. 7, I; and Plate III. fig. 8, I.

Back is the opposite side to that in which the aperture is placed or turned directly from the observer. Plate III. figs. 5 and 10, G G.

The venter, or belly, is the most prominent part of the lower whorl or body, generally situated in the vicinity of the lip over the aperture; and formed by the convexity of the aperture. It is in general only made use of in describing shells whose body is large in proportion to the size of the spire. Plate III. figs. 5 and 10, H H.

Sides. The extreme edges of the shell, when viewed either in front or from the back. Plate II. fig. 11, K K K K K K K K. Right side is, when the shell is viewed in front that side next the observer’s left hand. Left side, the side with the aperture in it.

Aperture, or mouth, is that part of the lower whorl or body by which the animal protrudes itself. This is one of the principal generic distinctions of Univalve shells, and differs very much in shape; some apertures being rounded, others semilunar, angular, &c. Plate II. figs. 9 and 11, C C; and Plate III. figs. 1, 3, and 14, C C C. Some apertures have a canal at their base, and others are devoid of it. In various genera it extends the whole length of the shell, as in the Cypræa, and some of the Cones with depressed spires. This in several individuals is either entirely open, or is closed by an operculum or lid, which is usually affixed to the foot of the animal.

Canal, or gutter, is the inside of the elongations of the aperture, or both lips of the shell of those species with a beak, in which it forms a concave channel or gutter, running from its commencement in the aperture to the extremity or base. Plate II. figs. 10 and 11, Q Q. Plate III. fig. 8, Q. Some species are furnished with two canals, one situated at the junction of the outer lip and body, as in the Murex.

Beak is that lengthened process in which the canal is situate; it commences a little higher up, on the outside, than the insertion of the canal in the inside, which is always distinctly marked by the line of the aperture. Plate II. fig. 11, p. This process is not so conspicuous in some of the species of Voluta, but is more marked in the genera Murex, Fusus, Pyrula, &c.

Pillar, or columella, is that process which runs through the centre of the shell in the inside from the base to the apex in most univalve shells, and appears to be the support of the spire: and, indeed, seems to form that part of the shell: it is in general grooved or folded; but, as it is situated in the interior of the shell, a minute description is unnecessary. Plate II. figs. 10 and 12, M M M M M; and Plate III. fig. 9, M M M. The internal edge of its base is frequently described as having plaits, &c.

Plaited Columella are those folds, or plaits, which are the distinguishing characteristic of the Volutæ and other genera. Plate II. fig. 7, z.

Pillar Lip is a continuation of the glossy process with which the aperture is lined, and expanded on the columella. Plate III. figures 7 and 8, O O. This is termed the inner lip by some authors.

Outer Lip. The expansion, or continuation of the body of the shell on the left margin of the aperture, and is also lined with the glossy process of the aperture. Plate II. fig. 7, N; and Plate III. fig. 8, N N N. The latter is an example of the alated or winged shells.

Operculum, or lid. This is only an appendage to the turbinated or spiral shells affixed to the foot of the animal, sometimes of a testaceous, in others of a horny or cartilaginous substance. It acts as a door or lid, and is calculated for the protection of the animal, when it retires within its dwelling, from the intrusion of its enemies, and adapted to the shape of the aperture, which it closes nicely up: as exemplified in the operculum of the Turbo, and is of a hard, stony appearance. Plate III. fig. 4.

Spire consists of all the whorls of the shell, except the lower one, which, as before observed, is termed the body of the shell. Plate II. fig. 12, D; and Plate III. figures 7 and 8, D D.

The spire is a prominent feature of the Univalve; and upon its being elevated, depressed, &c., depends much of the generic and specific definition. Adanson, in his ‘Natural History of Senegal,’ says that the external character of the spire varies according to the plane they turn upon, which, he observes, is either horizontal, cylindrical, conic, or ovoid. At the same time, he admits that there are a great many intermediate forms which cannot properly be defined.

It must be remembered that many of the young shells have not the same number of wreaths as the adults; from which it would appear, that the part of the animal nearest the apex never increases in size. The number of wreaths cannot, at all times, be depended upon. A full grown shell may, however, be known from the outer lip, which has generally an unfinished appearance in young shells. Indeed, in all the land and fresh water shells it is a distinct criterion, as they are never complete in the form of the outer lip till full grown.

Whorl is one of the wreaths or volutions of the shell. Plate II. fig. 8, L; and Plate III. fig. 10, L.

Depressed Spire is when the spire is very flat, as in the shells of the genus Planorbis, &c. Pl. II. fig. 12, D; and Pl. III. fig. 5, S.

A flat shell is figured in Plate III. fig. 14.

Involuted Spire, those shells which have their whorls, or wreaths, concealed in the inside of the first whorl or body, as in some of the Nautili and Cypræa.

Suture of the Spire, or whorls, is a fine spiral line, which separates the wreaths or whorls from each other; it is sometimes crenulated, undulated, or sulcated, and not unfrequently elevated or projecting. Plate II. fig. 9, E E.

Reversed, or Heterostrophe Spire, is when the volutions of the spire revolve in the same manner as a common corkscrew, or when the aperture is placed downwards, the nature of the spire runs upwards from the right hand to the left, Plate III. fig. 13.

In some of the more depressed species of Helix, or Nautilus, great attention is requisite in order to ascertain which is really the upper side of the shell, for it is on that side the spiral turns are to be taken from the centre or apex; and, in most instances, this is to be determined by the oblique direction of the aperture to the under part, where the lip rarely extends so far as on the upper part. In fixed shells, such as Serpulæ, there is no difficulty, as the side which is sessile must be considered as the base or under part. Thus, in the Serpula Lucida the fixed part is sometimes very small, and the mouth turns spirally upwards, in a contrary direction to the sun; and therefore must be considered a reversed or heterostrophe shell, the same as if the volutions nearest the mouth had turned laterally upon the centre or fixed ones. This shell, indeed, is most frequently found with regular lateral volutions; and though subject to great variety, with respect to contortions, it invariably turns the aperture one way.

In some species of Nautilus, however, there can be no rule to ascertain whether the shells are dextral or sinistral; for when the aperture is exactly central, the lip embraces the body equally, and the sides of the shell are similar.

Chambers are the cavities divided by partitions, at regular or irregular intervals; as in the Spirula. Plate III. fig. 11, W W W W W.

In some of the Serpulæ there are also divisions, but they are not regular as in the Nautili and Spirulæ; and besides, they differ from them in being devoid of a siphunculus or communication between the chambers, the animal forms a complete partition, and adds to its shell, which it would appear to be necessitated to do from its body growing too large for its abode.

Several of the Patellæ have chambers formed of laminous partitions, subspiral cells, or processes; these in general lie horizontally, and are quite open at one end, as in the Patella Testudinaria, the Crepidula, and Calyptræa.

Umbilicus is in general a circular perforation in the base of the lower whorl, or body, of many univalve shells. This is common to most of the Trochi, in some species of which it penetrates from the base to the apex; widest at the base, and gradually tapering to the top. Plate III. fig. 1, u.

Sub-umbilicated shells are those which have the umbilicus covered in a greater or less degree by a thin process; which, in some, almost entirely closes the opening or mouth. This character is most commonly to be met with among species of Buccinum and Murex.

Shells which have no umbilicus are termed imperforate.

Siphunculus is that small round perforation which forms a communication between the chambers of the Spirulæ, and penetrates through the whole spire of the shell. Plate III. fig. 11, v.

Varices are transverse ribs which cross the whorls of shells in some species of Buccinum, Murex, and Tritonia, and exemplified in the Cassidaria. Varices are formed by the periodical growth of the shells, these being the margin of the outer lip, to which the animal has attached its periodical enlargements. In some species they have more the form of sutures than ribs; this is owing to the margin of the outer lip being but slightly developed.

Ribs are those longitudinal and transverse protuberances which are in many of the univalve shells. Plate III. fig. 12, R R R R.

Teeth of univalves, or tooth-shaped protuberances, are fine, white laminæ, or ridges, running spirally backwards, in a parallel direction to each other; those on the exterior lip may, in most instances, be traced through the outside of the shell, and are nearly alike in length. Plate III. fig. 13, a.

Epidermis is a skin, or cuticle, covering the exterior surface of shells, destined by nature to protect their surface from being injured. It is membranaceous, somewhat similar to the periosteum which covers the bones of animals. This substance is the production of the animal inhabiting the shell; it is uniformly observed in some species, and not at all in others. Shells with a rugged or uneven surface have almost always this epidermis. In some it is strong, laminated, velvety, fibrous, or rough, often beset with long hairs, and in others very thin, smooth, and pellucid, and admits the colours of the shell to shine through it. In some species it is so dusky that it entirely obscures the beautiful colouring of the shell beneath. Although many shells are very beautiful, even with this cuticle on them, they are much more so when it is removed; but I would by no means advise the collector of shells to remove it, unless he may have several duplicates of the same species; and then he may do it by way of variety. It is always preferable, however, to keep them in a state of nature.

A shell with epidermis is represented in Plate III. fig. X; and the effect of the epidermis removed.

All other protuberances, furrows, &c., will be described at the end of this work in a Glossary of Terms used in the Science.