Obovaria olivaria (Rafinesque, 1820)
Other common names: Eggshell, chestnut shell, glassyback.
Key characters: A thick, rounded or oblong shell with a smooth surface, greenish or yellowish brown, shallow beak cavity.
Similar species: Round hickorynut, round pigtoe, ring pink, ebonyshell, Higgins eye, mucket.
Description: Shell oval or oblong, thick, solid, and inflated. Anterior and posterior ends broadly rounded. Umbos slightly elevated above the hinge line, rounded, curved inward, and directed forward. Beak sculpture of four or five delicate, double-looped bars, usually evident only in very small shells. Shell smooth, olive green or yellowish brown, faintly rayed in young shells, becoming dark brown in old individuals. Length to 4 inches (10.2 cm).
Pseudocardinal teeth triangular, relatively small, widely divergent, and horizontal. Lateral teeth straight to slightly curved, wide, heavy, and fairly long. Interdentum narrow. Beak cavity shallow. Nacre white, iridescent posteriorly.
Habitat: Large rivers (rarely in medium or small streams) in sand or mixed sand and gravel.
Status: Widespread and common in the Wabash and lower Ohio rivers but disappearing in the Mississippi and upper Ohio rivers. Endangered in Ohio. Species of Special Concern in Michigan. Watch List in Missouri.