Page:Natural History, Mollusca.djvu/285

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A group of fine shells, many of which are of large size, is thus named after the principal genus, Cyprina of Lamarck; though the appellation of the genus and that of the family are alike objectionable, as they have long been pre-occupied with the slightest possible terminal difference by the family and genus of the Carps among Fishes. The Mollusks of this group are distinguished by thick solid shells, often clothed with an epidermis, but usually smooth on the surface, or if sculptured, only with concentric lines running parallel to the edges of the valves. They vary in colouring, some being brilliant, others sombre. The hinge has strongly developed teeth, and an external ligament. The pallial impression is scarcely sinuated in its course. "This depends on a peculiar feature in the organization of the animal, which has, instead of distinct and produced siphonal tubes, only rudimentary ones in the shape of two scarcely separated orifices." The foot is thick and tongue-like.

This genus presents many points of affinity with the Cockles, and some with the great Clamps (Chama) of tropical seas. The shells are large, very convex, almost globose, heart-shaped, with the beaks much inrolled, curving forwards and outwards; the hinge is composed of two flattened teeth. The animal will be described presently.

The form of the valves has given the name of Heart-shell to the only British species, Isocardia