Idas simpsoni

(Marshall, 1900)

Shell is thin, fragile and equivalve with a smooth margin. Inequilateral, beaks in the anterior half. Outline oblong with curved anterior and posterior margins. Ligament external but deeply inset, stretching behind the umbones more than half-way to the posterior margin. Hinge line without teeth or crenulations. Sculpture of fine concentric lines; in the middle of the shell some short, faint radiating lines. Pallial line wide; adductor muscle scars about equal, but the posterior is extended dorsally (I. simpsoni-drawing).

Up to 23 x 9 mm.

Dirty white, periostracum like yellow-brown varnish, very glossy. Interior pearly white.

Trawled, deep water.

Off Shetland, Fair Isle, Aberdeen and SW Ireland, attached to whales' skulls (Distr. I. simpsoni).