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.