Arcidens confragosus (Say, 1829)
Other common names: Rockshell, grandmaw, bastard, black pocketbook, queen.
Key characters: Large, relatively thin shell, heavily sculptured, with poorly developed lateral teeth.
Similar species: Threeridge, washboard.
Description: Shell thin to moderately thick, elliptical, and inflated. Anterior end rounded, posterior end squared or bluntly pointed. Ventral margin straight or slightly rounded. Umbos full, elevated above the hinge line and located near the middle of the shell. Beak sculpture of large pronounced knobs or double-looped ridges that continue to the edge of the shell in two rows, developing into folds on the surface. Periostracum dark green, brown, or black. Length to 7 inches (17.8 cm).
Pseudocardinal teeth compressed, elongate; two in the left valve, one in the right. Poorly developed lateral teeth, often present only as a finely serrated thickening of the hinge line. Beak cavity moderately deep. Nacre white, iridescent on the posterior third.
Habitat: Medium to large rivers in pools and areas of reduced flow in mud and sand.
Status: Extirpated from Ohio. Threatened in Wisconsin. Rare in Missouri. Uncommon to rare in the other states of the Midwest.