Ptychobranchus fasciolaris (Rafinesque, 1820)
Other common names: None.
Key characters: Thick, kidney-shaped shell, heavy teeth, yellowish brown, with broken green rays.
Description: Shell solid, thick, elongate, compressed, and kidney-shaped. Anterior end rounded, posterior end bluntly pointed. Dorsal margin straight to slightly curved, ventral margin curved, straight, or arched. Umbos low, approximately even with the hinge line. Beak sculpture, if visible, of two or three indistinct ridges. Shell smooth. Periostracum yellowish brown to brown with broad green rays, usually interrupted or broken. Length to 6 inches (15.2 cm).
Pseudocardinal teeth thick and triangular; two in the left valve, one in the right, sometimes with a low, roughened tubercular tooth on either side. Lateral teeth short, nearly straight, heavy, and serrated. Beak cavity shallow or absent. Some females with a fold or groove on the inside. Nacre pearly white.
Habitat: Medium to large rivers in gravel.
Status: Fairly widespread but only locally abundant. Disappearing from parts of its range. Endangered in Illinois.