calcareous or horny pieces formed along the hinge line of boring bivalves (Pholadidae) to protect body when shell is in open position.

A secondary calcareous or corneous structure formed in some bivalves (e.g., Pholadidae) to protect the soft parts. In certain bivalves, a small shelly or partially shelly plate lying over the upper or lower margin, or a shelly plate attached to the internal ligament.