Elliptio dilatata (Rafinesque, 1820)
Other common names Lady finger.
Key characters Thick, elongate shell, short lateral teeth, greenish brown or black, usually with a purple nacre.
Description Shell solid, elongate, elliptical, and compressed to moderately inflated. Anterior end rounded, posterior end rounded to slightly pointed. Dorsal margin straight to slightly curved, ventral margin straight to curved in young shells, becoming arched in older shells. Umbos low, usually not elevated above the hinge line. Beak sculpture, if visible, of three or four heavy loops. Surface smooth, greenish brown with faint green rays visible on small shells, becoming dark brown to black in adults. Length to 5 inches (12.7 cm).
Pseudocardinal teeth well developed; two in the left valve, one in the right. Lateral teeth short, roughened, and straight. Beak cavity very shallow. Nacre variable, most often purple, occasionally pink or white.
Habitat Small to large streams and occasionally lakes in mud or gravel.
Status Widespread but sporadic in distribution. Common in Missouri and Ohio, uncommon to rare in other states. Species of Special Concern in Illinois.