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, becoming black in old individuals. Young shells typically marked with dark green rays that become fainter with age. Length to 8 inches.

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.