Potamilus alatus (Say, 1817)
Potamilus alatus, INHS 8049. Mackinaw River, McLean County, Illinois.
Other common names Purple heelsplitter, pancake, hatchet-back.
Key characters An elongate and rectangular shell, well-developed posterior wing, dark green to dark brown, with a purple or pink nacre.
Description Shell large, elongate, laterally compressed and somewhat rectangular, thin in young shells to moderately thick in older individuals. Anterior end rounded, posterior end bluntly squared or truncated. Umbos flattened and only slightly elevated above the hinge line. Beak sculpture of three or four concentric ridges, visible only in small shells. Large wing present posterior to the umbos. Shell smooth, dark green or brown, becomingblack in old individuals. Young shells typically marked with dark green rays that become fainter with age. Length to 8 inches (20.3 cm).
Pseudocardinal teeth small, roughened, thin and divergent; two in the left valve, two in the right. Lateral teeth long, thin, and straight to slightly curved. Beak cavity shallow. Nacre usually purple or pinkish purple, rarely white; highly iridescent.
Habitat Medium to large rivers in mud or mixed mud, sand, and gravel.
Status Widespread and relatively common.