(Müller, 1776)
Description:
C. anomala is rounded or somewhat squarely orbicular in vertical view, reaching about 15 mm in width. In lateral view, the dorsal valve is conical; the apex subcentral or near the posterior margin. Its surface is usually smooth or finely striate; concentric growth lines are present. It has neither a functional pedicle nor a pedicle opening; instead, the flattened ventral valve is cemented to the substratum. Its colour is very variable, the periostracum being dark brown or reddish, while the ventral valve is light grey. The shell is endopunctate. The valves are gaping in life, with approximately 60 non-retractile setae projecting in each anterio-lateral crescent.
Internal:
The animal is of white colour, tinged with yellow and brown. The lophophore consists of 2 lateral spirals, not supported by shelly material (C. anomala internal).
Habitat:
It is most commonly found in shallow to moderately deep water (15-165 m), although it has been collected from nearly 1500 metres. It lives attached to hard-substrata.
Distribution:
In the North Sea, the species occurs off Scotland and Shetlands. Elsewhere it is found from the Arctic to the Canary Islands.