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. P. undulatus. P. marmoratus. P. scriptus. P. angulatus P. stellatus. P. pallens. P. violacescens. P. zonalis. P. pennaceus. P. rubens. P. castaneus. P. pectiniformis. P. striatularis. P. nummarius. P. pectinatus. P. radians. P. vitreus. _P. glycimeris._ The delicious Pectunculus. Sub-orbicular, umbones produced; finely striated transversely and longitudinally; covered with epidermis, under which it is marked with reddish chestnut spots or bands; inside white; margins crenulated. _P. pilosus._ The hairy Pectunculus. Species convex, more or less smooth and hairy. _P. pectiniformis._ The Pecten-shaped Pectunculus. Pl. 11, fig. 6. Species lenticular, more compressed, pectinated, and more or less rough. 4. Nucula. Six species. Shell small, more or less thick, sub-triangular, equivalve, inequilateral; summits contiguous and turned forward; hinge similar, formed by a numerous series of very pointed teeth, pectinated and disposed in a line interrupted under the summit; ligament internal, short, inserted in a small oblique cavity in each valve; two muscular impressions; valves more or less pearly within.