_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. Cucullæa auriculifera. 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. Pectunculus pilosus. P. marmoratus. P. Pennaceus. P. castaneus. P. angulatus. P. palleus. P. zonalis. P. glycimeris. P. undulatus. P. scriptus. P. stellatus. P. violacescens. P. aggregatus. P. striatularis. P. pectinatus. P. rubeus. P. pectiniformis. P. nummarius. P. radiatus. P. vitreus. P. inscriptus. P. cinerosus. 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. Nucula lanceolata. N. pella. N. obliqua. N. elongata. N. tellinoides. N. crenifera. N. Arctica. N. curvirostra. N. glacialis. N. fluviatilis. N. minuta. N. para. N. Mauritania. N. rostrata. N. Nicobarica. N. Margaritacea. N. Costellata. N. gibbosa. N. eburnea. N. polita. N. nasuta. N. fabula. N. Elenensis. N. cuneata. N. striata. N. rugulosa. N. Limatula. N. lævis. N. concentrica. N. decussata. N. exigua. N. squamosa. N. gigantea. N. nitida. N. tenuis. N. convexa. N. Pisum. N. carinifera. N. torta. N. plicaria.