The two valves or a bivalve shell cover the right and left sides of the animal; they are hinged dorsally (above the body) and open ventrally (below the body). Usually the two valves are similar and equal in size, but in some forms, such as the oyster, that attach to the substratum by one valve (i.e., lying on their sides), the left-hand (or upper) valve is larger than the right-hand (or lower) one. Two muscles, called adductors, run between the inner surfaces of the two valves; acting antagonistically to the hinge ligament, they enable the shell to close rapidly and tightly.
Because of the enormous variety of sizes, shapes, surface sculpturing, and colors, shell characteristics are of great importance in the identification and classification of bivalves. Shells range in size from the tiny (1/16-in./2-mm) seed shells characteristic of members of the freshwater family Sphaeriidae to the giant clam, Tridacna, of the South Pacific, which attains a length of over 4 ft (120 cm) and may weigh over 500 lb (225 kg).