The shell is very large and triangular to hatchet-shaped. It is semi-translucent, thick, solid, and inflated. The dorsal margin is nearly straight, the posterior margin is broadly oval to truncate in outline, and the ventral margin is convex posteriorly and concave near the umbones. The exterior of the shell has 10 to 17 main radial ribs, often bearing scale-like spines and weaker interstitial riblets. The interior nacreous layer is strong, undivided, and occupies the anterior half of the valves. The hind margin of the posterior adductor scar slightly protrudes beyond the nacreous area. The color of the exterior is dark reddish-brown to nearly black, usually dull. The interior is dark brown to black, and the nacreous area is iridescent.