Opening and closing of shell is accomplished by adductor muscles which leaves two scars on each valve. Pelecypod shells vary greatly in size. Some are only few centimeters while other, like Tridacna are more than a metre wide. Cardinal area Dentition and Classification Pelecypod shells are hinged by means of teeth and sockets which together make up the dentition. Dentition in pelecypod shells is very important in their classification.
Taxodont dentition refers to numerous teeth that lie parallel to one another.
This type of dentition is shown by a group called palaeotaxodonts, eg. Dysodont dentition refers to simple teeth occurring near the edge of the hinge line. Cryptodonts either have dysodont dentition or lack teeth altogether. Heterodont dentition is present in shells with 2 - 3 large cardinal teeth and lateral teeth on both sides of cardinal teeth. Schizodont dentition has a median large tooth divided into two equal parts.