Marine bivalve mollusc. Shell solid, inequivalve, extremely rough, extensively fluted, and laminated; left (lower) valve deeply cupped, its sides sometimes almost vertical, the right (upper) valve flat or slightly convex sitting withing left; inequilateral, beaks and umbones often overgrown; tending to be oblong in outline but often disorted and very irregular. The shape of the shell varies with the environment. Colour usually whitish with many purple streaks and spots radiating away from the umbo. The interior of the shell is white, with a single muscle scar that is sometimes dark, but never purple or black.