Shell. Free, regular, inflated, equivalve, very inequilateral, with diverging summits, strongly flexed anteriorly and outwardly, in a commencing spiral, hinge dorsal, long, similar, formed of two flat cardinal teeth and one lamellous behind the ligament; ligament dorsal, exterior, diverging anteriorly towards the summit; muscular impressions very distant and small. Inhabits the Mediterranean and British seas. Four living species. One fossil.

FAMILY XI.
Arcacea. Four Genera.

1. Genus Arca. Pl. [VIII].

Animal. Body thick, slightly variable in form; abdomen provided with a pedunculated foot, compressed, fit for adhesion, and cleft throughout its extent; mantle supplied with a simple row of cirri and slightly prolonged posteriorly; buccal tentacula very small and very thin.

Shell. Somewhat diversiform, but most usually elongated and more or less oblique at the posterior extremity, often very inequilateral; summits more or less distant and little flexed to the front; hinge anomalous, straight, or a little flexed, long, and formed by a line of short vertical teeth, decreasing from the extremities to the centre; ligament exterior, wide, nearly as much before as behind the summit; two muscular impressions united by a band or palleal impression, not very distinct. Inhabits the American and British seas. Forty-one living species. Nine fossil.

2. Genus Cucullæa. Pl. [VIII].

Animal. As above.

Shell. Trapeziform, inequilateral, equivalve, heart-shaped; beaks far apart, separated by the angular groove of the ligament, which is altogether external; hinge linear, straight, with small transverse teeth, having at its extremity from two to five parallel ribs; valves minutely striated longitudinally; margins crenulated. The Cucullæa is distinguished from the Arca by the muscular impression, to one side of which is an auriform testaceous appendage; the shell also is more trapeziform. Inhabits the Indian ocean. One living species. One fossil.