The Bicolor Purse Oyster, Isognomon bicolor (C.B. Adams, 1852), has an oval, very flat shell, with thin-walled valves and coarse, wavy, blade-like lamellae. The shell can reach 40 mm (about 1.6 inches.) Internally the shell has a small nacreous area, the non-nacreous area is wide, and the hinge in juvenile shells has a set of ridge-like teeth. This species is known to grow on rocks and hard structures in many areas of the Florida Keys. The complete shell in the illustration was collected by Kimberly Nealon on Summerland Key, in April 2021.
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