Elongate, valves thick; numerous pustules scattered across valve, well developed posterior ridge; periostracum greenish to dark brown, rayless. Sexually dimorphic; females compressed and expanded posteriorly, males more truncated posteriorly. Teeth well developed, nacre white. Length to 8 inches. No similar species in Minnesota.
Distribution and Habitat.
In Minnesota, a species of large rivers in coarse substrate and flowing water. Rare in the Mississippi River below St. Anthony Falls, and extirpated from the Minnesota River. The best populations occur in the lower St. Croix River where it is common but rarely abundant.