Brittle shell up to 5cm in length. The shell is dark brown in colour but obscured by brown periostracum (tends to appear black in older specimens). The inside of the shell is pearly tinged with blue or brown. The surface is sculptured with obvious concentric lines crossed with ribs in the anterior and posterior regions. The margin of the shell is crenulate where the ribs are present but smooth elsewhere.
- There are 12 anterior ribs and 40-60 posterior ones.
- Periostracum is olive or brown in juveniles becoming black in adults
- 'Knobbly' appearance to the shell surface
Lives on muddy gravel bottoms where it filters phytoplankton.
LocalFrom the north of Norway to the south of Britain