They usually lie close to the sea floor. Fossil pelecypods are usually found with valves open or separate altogether. The shell consists of two valves are designated left and right, as opposed to brachial and pedicle valves in brachiopods. However, like brachiopods, they have beaks and hinge line along which the valves join. Both the valves usually have beaks.
Valves are generally convex.
The hinge line in one valve has teeth which fit into corresponding sockets in the other valve. Between the hinge line and the beak there is a flat or convex cardinal area . The line along which the mantle is attached to the interior of the shell is called the pallial sinus and is often preserved in fossils. Opening and closing of shell is accomplished by adductor muscles which leaves two scars on each valve. Pelecypod shells vary greatly in size.