Plethobasus cyphyus (Rafinesque, 1820)
Other common names: Bullhead.
Key characters: Oblong shell with a smooth surface except for a single row of bumps or knobs running from the umbo to the ventral margin.
Description: Shell thick, oval or oblong, somewhat elongate, and slightly inflated. Anterior end rounded, posterior end bluntly pointed. Dorsal margin straight, ventral margin curved anteriorly, straight posteriorly. Umbos slightly elevated above the hinge line. Beak sculpture of two heavy ridges, visible only in young shells. Shell smooth, except for a row of knobs or tubercles on the center of the valve, running from the umbo to the ventral margin (some- times obscure). A shallow sulcus or furrow present between the row of tubercles and the posterior ridge. Periostracum yellow or light brown in juveniles, becoming chestnut to dark brown in adults. Length to 5 inches (12.7 cm).
Pseudocardinal teeth rather small relative to overall shell size; two in the left valve, one in the right (occasionally with a smaller tubercular tooth on either side). Lateral teeth long, straight or slightly curved; two in the left valve, one in the right. Beak cavity shallow. Nacre white, occasionally tinged with pink or salmon.
Habitat: Medium to large rivers in gravel or mixed sand and gravel.
Status: Rare throughout its range. Endangered in Indiana, Iowa, Ohio, and Wisconsin. Threatened in Illinois. Rare in Missouri.