Shell size to 70 mm; shell elongate, thin, fragile. Valves widely gaping. Sculpture of radial ribs and concentric ridges. Anterior ridges with scale-like projections. Radial ribs weak or lacking posteriorly. Color dull-white. The Atlantic Mud Piddock bores into hard clay and soft rock. It will settle as a larva onto the host rock, and will grow as it bores into it. The resulting borehole is conical (with the narrow end toward the rock surface), and the piddock ends trapped inside the rock for life.