Species in this family (Lyonsiidae) have no hinge teeth. This is the hinge on the left valve. Notice the calcified shieldlike plate that wraps around the hinge ligament from the left valve.
The shell is fairly inflated, as can be seen in this end-on view of the left valve. The shieldlike plate which wraps around the hinge ligament can also be seen.
The shell is thin, and along the ventral edges is mostly yellowish-brown periostracum. In this view of a live individual, the thin, flexible periostracum along the ventral margin has distorted when the animal closed, forming into two nearly parallel plates.
