outline is more triangular, and their surface is smooth, or merely marked with fine concentric lines. They are generally swollen; their substance varies much in texture; they are often invested with a strong epidermis. "The valves are connected together by a hinge, consisting of a forked diverging tooth in one, raised on a ligamental fulcrum lodged in a cavity, which is marginated in the other, a connecting cartilage and small external ligament completing the union." The pallial impression closely resembles that of the Veneridæ.
The animals of this family differ in the degree in which the mantle is open or closed in front; there are two siphonal tubes, which are connected and fringed at their orifices, and sometimes studded with pointed warts.
Genus Mactra.
The shell is more or less triangular, slightly gaping, with a smooth or concentrically striated surface, covered with an epidermis. The hinge is composed of a V-shaped cardinal tooth in one valve, locking into a marginated pit in the other, and of a long lateral tooth on each side of the same valve, which also fits into a deep groove with tooth-like margins. The sinus of the pallial impression is shallow but wide.
The animal has the mantle open as far as the siphons, its edges are fringed; the foot is strong, tongue-shaped and bent, capable of protrusion to a considerable extent. It is used, as in the Cockles, for burrowing in sand, which these animals do with great ease and rapidity. They are inhab-
