A bivalve shell is the enveloping exoskeleton or shell of a bivalve mollusc, composed of two hinged halves or valves. The two half-shells, called the "right valve" and "left valve", are joined by a ligament and usually articulate with one another using structures known as "teeth" which are situated along the hinge line. In many bivalve shells, the two valves are symmetrical along the hinge line — when truly symmetrical, such an animal is said to be equivalved; if the valves vary from each other in size or shape, inequivalved. If symmetrical front-to-back, the valves are said to be equilateral, and are otherwise considered inequilateral.
The bivalve shell not only serves as protection from predators and physical damage, but also for adductor muscle attachment, which can allow the mollusc to "swim" short distances by flapping the valves. The shell is secreted by a soft part of the molluscan body known as the mantle and has several layers, typically made of calcium carbonate precipitated out into an organic matrix.