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A MULTIVALVE shell is composed of more parts than two. Every part of a shell which is connected by a cartilage, ligament, hinge, or tooth, is called a valve of such shell; thus, the Chitons have eight transverse, broad, but very short valves, placed on the back of the animal, and inserted at their sides into a marginal tough ligament. Plate I. fig. 11, a a a.

Operculum consists of four small valves on the summit of the Lepas, which shut up the superior orifice; it is in a certain degree stationary, and different from the operculum of univalve shells, which will hereafter be described. Plate I. fig. 1, A. Fig. 9 represents a profile view of the operculum removed from its place, a the front valves, b the back valves. Fig. 10 a front view of the operculum.

Base is that part of the shell by which it is fixed to rocks and other bodies. Plate I. fig. 1 and 2, B B B.—H is a piece of stone to which the base is fixed, and G a piece of wood to which the shells of this section are generally attached.

Ligament is the membranous or tendinous substance by which the valves or parts of the shell are attached. Some multivalve shells are connected by the parts of one valve locking into another. Plate I. fig. 2, D D D. The ligaments vary considerably in their texture, being scaly, prickly, smooth, or punctated.

Ridges are certain convexities in many of the Lepas tribe, sometimes longitudinal and sometimes transverse. Plate I. fig. 1, F F.

Peduncle. A sort of stem by which the shells of the Anatifera are attached to wood, &c. It is a membranaceous substance, similar to a bladder but materially thinner, and filled with a liquid which evidently affords nourishment to the animal. Plate I. fig. 2, C C; the peduncle is usually affixed to a piece of wood as represented at G.

Feelers are those crenated arms, evolved from the side of the Anatifera. While the animal is in the water it continually moves its feelers, evidently for the purpose of entangling minute marine insects, as food. Plate I. fig. 2, E E.

Accessory valves are small plates which cover the apex at the hinge of the Pholades, or are situated below the hinge. Plate I. fig. 3, a.

Margin. A fleshy border in which the valves are attached in the genus Chiton. Plate I. fig. 11, b b.