Species elongated, very inequilateral; summit rounded and recurved anteriorly; two short divergent cardinal teeth, besides a lamellous tooth; ligament very long, projecting or not; yellowish white, covered with decussated striæ. 4. Hiatella. Two species. Established by Daudin; classed by Linnæus with the Solen, but Lamarck is of opinion that it more nearly approximates the Cardita. Shell thin, sub-rhomboidal, equivalve, very inequilateral, gaping at its inferior margin and posterior extremity; the summit very anterior and recurved in front; dorsal hinge formed by a single tooth on one valve corresponding to a semicircular slope on the opposite valve, or by a small tooth with a cardinal cavity in each valve; ligament probably exterior and dorsal; muscular impressions unknown. Hiatella Arctica. Hiatella biaperta. _H. Arctica._ The Arctic Hiatella. Shell small, transversely oblong; apices truncated, with two divergent spring ridges; a small tooth on each valve; cream colour, with decussated striæ; inside pearly. _H. biaperta._ The double-clefted Hiatella. Pl. 10, fig. 4. Species that has only a single tooth on one valve; yellowish white. 5. Isocardia. Three species. Taken from the Chama of Linnæus on account of a peculiarity in the shape of the cardinal teeth, and the singular curvature of the umbones. Isocardia cor. I. semisulcata. I. Moltkiana. _I. cor._ The heart Isocardia. Pl. 12, fig. 4. Shell free, regular, heart-shaped, equivalve, very inequilateral; summits diverging, strongly recurved spirally, forward, and outward; hinge dorsal, long, similar, formed by two flat cardinal teeth, with an elongated lateral one behind the ligament, which is dorsal and exterior, diverging towards the summits; muscular impressions very distinct and rather small; slightly wrinkled longitudinally; exterior reddish chestnut colour, interior white. The Isocardia Moltkiana is a very rare shell, and the most elegant species of this genus. FAMILY XI. ARCACEA. Four genera. 1. Cucullæa. One species. Distinguished from the Arca by the muscular impression within, to one side of which is an ear-shaped testaceous appendage; the shell is more trapeziform, and the hinge by age becomes obsolete, giving the teeth a more horizontal appearance. Shell equivalve, inequilateral, trapeziform, heart-shaped; beaks distant, and 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 marked with minute and strong longitudinal striæ, and sometimes one valve overlaps; margins crenulated. _C. auriculifera._ The eared Cucullæa. Pl. 10, fig. 1.