"In the shells of the second division, Bivalves, we shall find a greater number of parts. Valves are the different pieces that compose a shell. When both the valves are alike in form, the shell is called equivalve: when the valves are different in the same shell, it is called inequivalve. Mya, Solen, Tellina, are equivalves: Ostrea, Anomia, Pinna, &c. are inequivalves.
TERMS—
BIVALVES.
"The hinge is formed by the teeth of one valve inserting themselves between those of the other valve, in some genera; in others, by the teeth fitting into the cavities of the other valve ([Plate 3.], a.) When the teeth are placed in the centre of the hinge they are called cardinal teeth. Lateral teeth are situated on the sides of the valves, and are generally long and flat, sometimes hollow. Some hinges are straight, others curved. Here is the hinge of Arca, furnished with many small teeth. ([Plate 3], b.)
"The ligament is a membrane that connects the valves, and keeps the hinge in its proper place: it is always situated near the beaks. The ligament is very perceptible in the cockle, in Pecten, or scallop, in Tellina, &c.
"The beaks are the most pointed parts of the bivalve shell ([Plate 3], c.); when the valves are closed, the line where they meet is called the seam. ([Plate 3], d.)
"The anterior slope is that part of the shell where the ligament is placed, and is also called the area. ([Plate 3], e.) The posterior slope, or areola, is the other side of the beaks. (f.)
"The lunula is a crescent-like depression on either the area or areola. The edge of the valve is called the margin; it is often finely crenulated, or toothed. The interior of the valve is called the cavity. (g.) In the valves of this ark-shell here are two broad marks, shining and glossy. (g.) In those of the oyster and muscle that I now show you, there is but one. These marks are muscular impressions; they are the parts where the muscles of the animal have been affixed, and are termed cicatrix.