 The two valves of the bivalve shell are held together at the hinge by a ligament composed of two keratinised proteins , tensilium and resilium . In different groups of bivalves , the ligament may be internal or external in position . The main function of the ligament ( as well as joining the valves together ) is to passively cause the shell to open . The shell is actively closed using the adductor muscle or muscles which are attached to the inner surface of both valves . The position of the muscles is often clearly visible on the inside of empty valves as circular or oval muscle scars . Along the hinge line of the shell are , in most cases , a number of hinge teeth which prevent the valves from moving laterally relative to one another . The arrangement of these teeth is often important in identifying bivalves . 
