Species boat-shaped, oblong, striated transversely and ribbed longitudinally; umbones remote and incurvated; margins entire and gaping; hinge straight; whitish, with divergent zigzag chestnut stripes; inside bluish white.
A. tortuosa. The twisted Ark.
A rare species; shell elongated, close, twisted; hinge completely straight.
A. barbata. The bearded Ark.
Species with the hinge straight, not hollowed or not gaping inferiorly, and of which the muscle is not adherent.
3. Pectunculus. Nineteen species.
This genus has the ligament partially inserted internally, and has no exterior angular groove. The valves never gape, often have rayed longitudinal ribs, are compressed, and the shell by age becomes thick and ponderous, sometimes attaining a large size. The teeth in the hinge are not so numerous as in the Arca and Cucullæa; the centre teeth appear worn down.
Shell close, orbicular, doubly convex, equivalve, sub-equilateral; summits almost vertical, and more or less distant; hinge formed on each valve by a rather numerous series of small teeth, disposed in a curved line, sometimes broken under the summit; ligament external and large.
Pectunculus glycimeris.
P. pilosus.