Cockle shell. Coloured scanning electron micrograph (SEM) of the shell of a cockle, a bivalve mollusc. Bivalve molluscs are so-called because they have symmetrical two-part shells joined by a hinge. Other common bivalve molluscs include scallops, clams and oysters. They are found in the sea and in fresh water. The shell is usually formed of the mineral calcite, sometimes with other minerals involved as well. The shell protects the soft body of the mollusc inside. A ligament controls the opening and closing of the shell.