(Searles-Wood, 1841)
Description:
Shell sub-rectangular in outline, as wide or wider than long. It is slightly bilobed anteriorly and reaches a maximum length of 2 x 3 mm. It has a wide straight hinge-line. The external surfaces are smooth, save for faint concentric growth lines. The shell is endopunctate. Dorsal valve D-shaped, with the posterior margin straight; moderately convex. Ventral valve deeper, with a straight umbo. The foramen is large and incomplete; the deltidial plates do not join medianly. It is white, yellow or grey in colour.
Internal:
Median septum present; the brachial loop comprising two semicircles from the hinge to the anterior end of the septum. Lophophore bilobed (A. cistellula internal).
Habitat:
It lives attached to hard-substrata, in shallow to moderately deep water (20-82 m).
Distribution:
The species occurs off north-east Scotland, Norway, English Channel, west coast of the British Isles, Sardinia and Sicily.