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.
Species navicular or obliquely heart-shaped, with decussated striæ; hinge completely straight, with two parallel ribs at each end, the terminal teeth longer and more oblique than the others; exterior chestnut colour, interior white, tinged with violet.