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.
Palaeoheterodonts have heterodont or schizodont dentition. In desmodont dentition, teeth are very small or missing. Heterodonts have either heterodont or desonodont dentition. Evolution First fossil records of pelecypods are found in Ordovician . They became abundant during Silurian and continued to grow steadily thereafter and reached climax of development in the Quarternary .