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Length of the Shell is taken from the ligament, or the beak, to the opposite margin. For example, Mytilus: it is longer than it is broad, and the Solens are broader than long. Plate I. fig. 5, u u; and Plate II. fig. 4, v v.

Breadth is measured from the most extreme edge of the anterior and posterior slopes, being in a contrary direction from its length. Many shells are broader than long, such as most of the Myæ, Solenæ, Tellinæ, &c.; and the Mytilus, Ostrea, Pinna, &c., are in general longer than broad. Plate I. fig. 5, v v. Plate II. fig. 4, v v.

Byssus, or beard, is an appendage composed of filaments of a silky texture, by which some of the Bivalves fasten themselves to their beds, such as the Mytilus, Pinnæ, &c. Plate II. fig. 6, s.

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The shells called univalve, or those composed of one part only, are far more numerous than the two preceding, both in genera and species; and it requires a considerable degree of attention to discriminate many of the species, as they run into each other so much; and they are divested of the strong and distinct character afforded by the teeth of bivalves; besides many of the species there are several varieties.