Conchæ Marinæ. Four Genera.

1. Cyprina. Pl. VII.

Animal. Thick, oval, foot compressed, falciform, geniculated mantle closed behind, and pierced by two oval apertures with cirrous edges: no veritable tubes.

Shell. With epidermis; thick, regular, substriated longitudinally, sub-cordiform, equivalved, inequilateral; summits strongly flexed to the front and often contiguous; hinge thick, subsimilar, formed by three slightly converging cardinal teeth, and by a posterior lateral tooth, sometimes obsolete; ligament very thick; muscular impressions distant, subcircular, and united by a narrow marginal band. Inhabits the Atlantic Ocean and British seas. Two living species. Seven fossil.

2. Genus Cytherea. Pl. VII.

Animal. Oval or round, generally but little compressed; edges of the mantle undulous, and garnished with tentacular cirri in one row; foot considerable, compressed, trenchant, in other respects diversiform; tubes tolerably elongated, and most usually united; mouth small; labial appendages quite small; branchiæ wide, short, free, or not united either with one another or with those of the opposite side.

Shell. Solid, equivalve, regular, inequilateral; summits equal, reflexed, and slightly projecting; four primary teeth on one valve, of which three are divergent, and approximating at the base, and one remote—this circumstance easily distinguishing it from the Venus. On the other valve are three primary divergent teeth with a distant cavity parallel with the edge. Inhabits the British, Mediterranean, and American seas. Eighty-three living species. Nine fossil.

3. Genus Venus. Pl. VII.

Animal. As above.

Shell. Solid, thick, regular, perfectly equivalved and close, more or less inequilateral; summits well marked and inclined to the front; hinge subsimilar; the middle cardinal tooth forked, or three cardinal teeth more or less contiguous and convergent towards the summits; ligament thick, often arcuated, convex, exterior; two distant muscular impressions; cordiform depressions beneath the beaks. Inhabits the British and American seas. One hundred living species. Nine fossil.

4. Genus Venericardia. Pl. VII.

Animal. Nearly as above.

Shell. Suborbicular, inequilateral, equivalve, sides having usually longitudinally rayed ribs; hinge with two oblique cardinal teeth in each valve, turned in the same direction. Five living species. Ten fossil species.