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.