Shell. Covered with a thick epidermis, enclosing it entirely except at the extremities, regular, somewhat thick, oval, elongated, straight and parallel edges, also rounded at its two extremities, valves equal, very inequilateral; the anterior side much longer than the posterior; summits feebly marked; hinge subsimilar, formed by a cardinal tooth, dilated, compressed, and a little reflexed above; subexterior ligament inserted upon the tooth, and nearly at the posterior extremity of the shell; two small, rounded, widely separated muscular impressions, without any visible abdominal impression. Four species.

7. Genus Amphidesma. Pl. [VI].

Animal. Very imperfectly known.

Shell. Transverse, inequilateral, somewhat rounded or suboval; sides slightly gaping; hinge with one or two cardinal teeth, and a narrow groove for the internal ligament; ligament double, the external one short, the internal one fixed in the internal grooves. The two ligaments distinguish this genus from all other bivalves. Inhabits the West Indian and American seas. Forty-five species.

FAMILY VI.
Corbulacea. Two genera.

1. Genus Corbula. Pl. [VI].

Animal. Unknown.

Shell. Tolerably solid, regular, inequivalve, inequilateral with a conical, flexed, ascending, primary tooth in each valve; a cavity at the side; no lateral teeth; ligament interior, placed in the cavities; two neighbouring muscular impressions. Inhabits the British and American seas. Ten species. Four fossil.