 Near the hinge of the shell is the umbo , often a rounded , knob @-@ like protuberance usually surrounding the beak . The umbo generally and the beak specifically represent the oldest portion of the shell , with extra material gradually being laid down along the margins on the opposite sides . The hinge point or line is the dorsal region of the shell , and the lower , curved margin is the ventral region . The anterior or front of the shell is where the byssus ( when present ) and foot are located , and the posterior of the shell is where the siphons are located . With the umbones / hinge uppermost and with the anterior edge of the animal towards the viewer 's left , the valve facing the viewer is the left valve and the opposing valve the right . 
