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Cavity of the Hinge. The hollow depression in which the ligament of the Ostrea is situated, generally of a triangular form. Plate IV. fig. 11, g.

Ligament of the Hinge, or cartilage, is that flexible fibrous substance by which the valves are united, and the hinges kept in their proper places; generally situated under the beaks of the shell. Plate II. fig. 1, 3, and 4, l l l; Plate IV. fig. 3, n.

Beak is the extreme point of the summit of bivalves, which in many species turns spirally downwards or to the one side, as in some species of Venus, &c. From this circumstance it is seldom the highest part of the shell. Plate II. fig. 5, r r.

Seam. When the valves are closed, the line of separation between them is so called. Plate II. fig. 6, t t.

Umbo. That part situated immediately under the beak. Plate I. fig. 6, w; and Plate II. fig. 4, w.

Ears. The processes on each side of the beak, in most species of that division of Ostrea, called Pecten; some have one ear very large, and the other small; and some are scarcely observable on one side. Pl. II. fig. 2, h h.