 The shell of a bivalve is composed of calcium carbonate , and consists of two , usually similar , parts called valves . These are joined together along one edge ( the hinge line ) by a flexible ligament that , usually in conjunction with interlocking " teeth " on each of the valves , forms the hinge . This arrangement allows the shell to be opened and closed without the two halves detaching . The shell is typically bilaterally symmetrical , with the hinge lying in the sagittal plane . Adult shell sizes of bivalves vary from fractions of a millimetre to over a metre in length , but the majority of species do not exceed 10 cm ( 4 in ) . 
