Obovaria subrotunda (Rafinesque, 1820)
Other common names: None.
Key characters: Almost perfectly round, relatively small shell, with a smooth, brown, rayless periostracum.
Similar species: Hickorynut, round pigtoe, ebonyshell.
Description: Shell round or circular, moderately thick, and inflated. Anterior and posterior ends rounded in males, somewhat truncated in females. Umbos low and centrally placed, slightly elevated above the hinge line. Beak sculpture of a few indistinct, concentric ridges, usually evident only on very young shells. Shell smooth, growth lines dark but often indistinct. Periostracum yellowish brown to dark chestnut brown or blackish, and rayless. Length to 3 inches (7.6 cm).
Pseudocardinal teeth moderately small, serrated, and divergent. Lateral teeth slightly curved, fairly short, and moderately thin. Interdentum narrow or absent. Beak cavity fairly deep. Nacre white, iridescent posteriorly.
Habitat: Medium-sized streams in sand and gravel in areas with moderate flow.
Status: Sporadic in distribution, disappearing from many areas. Endangered in Illinois and Michigan.