Generalized shape; slightly ovate, simple outline with large diamond-shaped orifice. Plates are smooth and thin. Radii and alae are nearly parallel with the base. Scutum is large with broad colour stripe radiating from the umbo. Tergoscutal flaps are white, sometimes tinged yellow or pink, with three black or purple spots. The basis is calcareous and porous.
Medium sized; up to 10 (15+) mm.
Plates pearly or grey, each with several purplish to dark brown stripes.
Open seas and estuaries, intertidal and sublittoral. On ships and buoys, piers, etc., especially in docks warmed by generating station effluents.
In British Isles only as introduced or fouling species. Elsewhere: Europe, West Africa, Mediterranean, Black Sea. Several closely related species worldwide.
A tropical form.