Shell solid, tumid and inequivalve; left valve slightly more convex than the right. Inequilateral, beaks in front of the midline. Irregularly rhomboidal in outline, set somewhat obliquely with the curve of the anterior border sharper than the posterior. Ligament in numerous grooves across the cardinal area, grouped like an elongate diamond. Sculpture of fine concentric and radiating ridges giving a reticulate effect to the surface. Growth stages are not clear. Hinge plate with a large number of very small similar teeth in a straight line with 3-4 larger teeth set obliquely to this row in two groups at each end of it; these larger teeth have serrated surfaces. The small teeth appear very like a crenulated margin and are often worn away (B. pectunculoides).
Up to 4.8 mm in length.
Dirty white. Periostracum straw coloured. Inside of shell is white.
A rare bivalve in the North Sea (Distr. B. pectunculoides). It is widely distributed over the North Atlantic, from west Greenland, south of Iceland and the Faroes to the Mediterranean, the Atlantic coast of Morocco, the Azores and to the W. Indies. Several varieties of the species have been described and it is difficult to establish which of them are valid.