3. Genus Pectunculus. Pl. [VIII].

Animal. Body round, more or less compressed; mantle without cirri or tubes; foot securiform, cleft at its inferior anterior edge; buccal appendages linear.

Shell. Orbicular, equivalve, subequilateral; summit nearly vertical, and more or less distant; hinge formed upon each valve, of a numerous series of small teeth disposed in a curved line, sometimes interrupted beneath the summit; ligament as in Arca, but usually much narrower. Inhabits the British and Mediterranean seas. Twenty-two living species. Nine fossil.

4. Genus Nucula. Pl. [VIII].

Animal. Body subtriquetral; mantle open in its inferior half only, with whole edges, denticulated throughout the extent of the back, without posterior prolongation; foot very large, thin at the root, enlarged into a wide oval disk, the edges of which are furnished with tentacular digitations; anterior buccal appendages, pretty long, pointed, stiff, and applied one against the other like jaws; the posterior ones also stiff and vertical.

Shell. More or less thick, subtriquetral, equivalved, inequilateral, with summits contiguous and inclined anteriorly; hinge similar, formed by a numerous series of very sharp teeth, pectinated and arranged in a line interrupted under the summit; ligament internal, short, inserted in a little oblique pit in each valve; two muscular impressions. Inhabits the British and American seas. Forty-five living species. Four fossil.

FAMILY XII.
Trigonacea. Two genera.