Surf'aces qf the valves.—Every bivalve shell has two surfaces, an external surface and an internal surface. Various parts are distinT guished on the external surface,—the hooks, the belly, the edges, the lunule, and the corslet.
External surface. . The hooks. — The protuberant opposed parts, often inclined to wards the anterior side, and presenting an apex of various degrees of sharpness or blunt ness, are thus denominated. When these hooks are very much inclined forwards, they are styled lateral. If they are particularly prominent, they are said to be cordiform. When they are inclined towards one another, so that their summits approximate, they are said to be opposed. The hooks in no case in cline to the posterior side ; but occasionally they disappear almost entirely, and, as in the Solens, exhibit no kind of prominence. In other instances again they project a great way, and form the most prominent part of the shell Pinna), in which case the books are said to be terminal.
2. The belly of the shell comprises the greatest part of the exterior surface. It is bounded at the base of the hook, as also by the lunule and the corslet. It is more or less rounded or flattened according to the general form of the shell, and we shall speak of the dif ferent points worthy of consideration con nected with it when we come to define the various particulars of the external surface con sidered in a general manner.
3. The edges. — These are indicated, pre serving the shell in the position which we have mentioned ; they are anterior, posterior, inferior, and superior. The extent of these edges is very various, and depends entirely on the form of. the shell and the position of its hooks. Upon this particular the simple in spection of a collection of shells will give much more information than we can hope to do by the most laboured description ; so that we shall only say that the anterior edge cor responds to the head of the animal, the pos terior to its posterior extremity, the inferior to its ventral aspect, and the superior to its back. The edges in themselves, however, present a few particulars which it were well to mention.
Sometimes they are thin and cutting; very com monly too they are thick and continue simple; In those species especially whose shells are marked externally with longitudinal strix, they are notched and toothed alternately, the pro jections of the one valve in almost all instances being received into the cavities of the other. When these projections and notches are very fine, the shell is said to be crenate; if larger, toothed; when very large and few in number, with their summits blunt, again, the edge is undulating as in the Tridacna; on the con tmry it is angular when the prominences con tinue sharp as they do in certain of the.Ostrew; in the latter case the edge is also said to be widely or deeply dentated.
' 4. The lunula does not occur in every genus of bivalve shell. It is met with, how ever, among the greater number of the Mono myaria; it is also met with among many Dy myaria, and is particularly conspicuous in the Venus. It is a space comprised. on the ante rior surface immediately under the hooks, and is generally circumscribed by a particular line. or depression. -The lunula presents certain pe culiarities which it is often of consequence to attend to, in order to distinguish certain spe cies otherwise apt to be confounded with one another. Its form is various, sometimes cordi form, a shape which almost peculiarly belongs to inflated and subglobular species; sometimes lanceolate, sometimes very narrow, especially in species whose shells are flattened. The lunula is very rarely prominent, unless it be towards the centre. Sometimes it is super ficial, pretty frequently depressed, and there are a few genera or species in which it is deeply hollowed.
5. The corslet occupies a part of the su- . perior and posterior edge of the shell. It is only met with in the Dimyaria ; it is not so accurately bounded as the lunula ; it is also wanting in a great number of genera, in which its presumed position is arbitrarily determined. It is towards its upper part that the nymph are observed in those species whose ligament is external.