Muscular impression is the impression left on the inside of the valves, by the adhering muscles of the animal. It differs in most shells, according to the shape of the animal, as semi-ovate, round, lunate, elongated, &c. As a specific distinction, it is often of great use; being, with a very few exceptions, alike in shells of the same species. Some shells have only one cicatrix, as the Edible Oyster and Mytilus; others have two, and some few more; the Tellina for example. Plate [I]. fig. 6 and 8, e e e e.
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.
Primary teeth are those teeth in general situated in the centre of the hinge, and are for the most part broad, large, and distinct, often elevated; and in general are inserted in a cavity in the opposite valve. They however differ very much in some shells, but may easily be distinguished. Plate [IV]. fig. 1, 3, and 7, a a a a a. These are also termed the Cardinal Teeth.
Lateral Teeth, Plate [IV]. fig. 10, are teeth which diverge from the Umbo, and are in general long and flat, often double and divided by a groove or hollow. Plate [IV]. fig. 2, 4, 6, and 7. b b b b.
Double Teeth, Plate [IV]. fig. 1, c c.
Incurved Teeth are those which are bent round, as in the single tooth of the Solen. Plate [IV]. fig. 8, f f.