Hinge teeth

Bivalves by definition have two valves, which are joined together by a strong and flexible ligament situated on the hinge line at the dorsal edge of the shell.

In life, the shell needs to be able to open slightly to allow the foot and siphons to protrude, and then close again, without the valves moving out of alignment with one another.

To make this possible, in most cases the two valves are articulated using an arrangement of structures known as hinge teeth (often referred to collectively as the "dentition").

Historically the hinge teeth have provided a convenient means by which to construct classification schemes and attempt to indicate the phylogenetic relationships within the class Bivalvia.

The anodont hinge is characterized by a strong ligament (or a series of transverse ligamental grooves as in the Isognomonidae), however, true teeth are absent in adults as is the case in the pen shells (Pinnidae), tree oysters (Isognomonidae), the pearl oysters (Pteriidae), and some freshwater mussels such as the genus Anodonta.