Marine bivalve mollusk. The shell is solid, unequivalve, extremely rough, fluted and laminated; left (lower) valve deeply concave with sides sometimes nearly vertical; the right (upper) valve is flat or slightly convex, resting inside the left one; the sides are uneven, with protruding bills and umbos, tending to be oblong but are highly distorted and irregular. The shape of the shell varies with the environment. The color is usually whitish with purple striae and points radiating from the umbo. The interior of the shell is white, with a single muscle that is sometimes dark, but never black.