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Lunule. The lunated depressions, situated in the anterior and posterior slopes. In different species of Venus they are prominent characteristic marks, often of much service in ascertaining a species. Plate I. fig. 4, b b.

Ligament perforation. The circular aperture, or perforation through which the ligament passes; by which the animal of the Anomia attaches itself to stones and other marine extraneous bodies, it is in general situated in the flat valve, though there are a few exceptions to the contrary. Plate I. fig. 7, m.

Hinge is the point at which bivalve shells are united; it is formed by the teeth of one valve inserting themselves between those of the other, or by the teeth of one valve fitting into the cavities or sockets of the opposite one.

It is on the peculiar construction of the hinge that the generic character of bivalve shells is principally founded, together with the general contour of the shell. Plate II. fig. 1 and 3, q q q.

Teeth of the hinge. Upon the number and relative situation of the teeth principally depend the specific distinctions; they are of various forms, and very differently placed; some are single and large, others numerous and small, orbicular, spatuliform, laminated, &c. Some hinges have no visible teeth, and are termed inarticulate.—When a primary tooth has a groove or hollow in its centre, it is called complicated. Plate IV. Fig. 6, e; those with few teeth are termed articulate. Plate IV. fig. 1 and 12; and those with many teeth multiarticulate. Plate IV. fig. 2, 3, 4, &c. Plate I. fig. 6, and 8, g g g g.