Shell length to 30 mm; moderately compressed, left valve more convex than the right. Sculpture: left valve with numerous low radial ribs and ridges, minutely scaled; right valve similarly sculptured but more profusely scaled. Colour: variable, usually purple or red, variably patterned with purple or black, and irregular white concentric banding. Habitat: attached under fallen coral plates on hard substrate. Distribution: Great Barrier Reef, central to north Queensland and offshore reefs, north Western Australia.