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.

Conchæ Fluviatiles. Three Genera.

5. Genus Cyclas. Pl. [VII].

Animal. Body oval, thick; edges of the mantle simple; tubes short and united; foot wide, compressed at base, and terminated by a sort of leg or appendage.