Hinge teeth

Hinge teeth are part of the inner surface of the shell of a bivalve mollusc. Bivalves by definition have two valves (parts of the shell). They are joined together by a strong and flexible ligament on the hinge line at the dorsal (top) 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, the two valves usually have hinge teeth (the "dentition"). Like the ligament, the hinge teeth are along the hinge line of the shell.
In most families, the two valves of the shell are almost perfectly symmetrical with one another along the hinge line, though the placement and shape of the teeth may differ slightly in the left valve and right valve so the two valves fit together properly.
Each group of bivalves tends to have distinctive hinge teeth. Because of this, examining the arrangement of the hinge teeth in a bivalve shell is often essential for identification and classification.
Brachiopods.
The other great shelled phylum, the brachiopods, also has a group with hinge teeth. In the traditional classification, the Articulata have toothed hinges between the valves. In the Inarticulata the two parts of the shell are held together only by muscles.87–93