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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.

Recurved Teeth, are those which are bent backwards, as in the hinge of the Panopea and Spondylus.

Middle Teeth, Plate IV. fig. 4, d d. These are also termed Cardinal Teeth.

Numerous Teeth, are those small upright teeth, set in rows, of which the hinges of all the species of the genus Arca are formed. Plate IV. fig. 10, k k.