Irus irus

(Linné, 1758)

Shell elongate, oblong, or irregular, umbones anterior to midline. Sculpture of about ten thin, concentric ridges, raised and convoluted, conspicuous posteriorly where they form distinct frills. Fine radiating ridges visible between the concentric sculpture. Growth stages are indistinct. Lunule is obscured, escutcheon elongate, narrow, most prominent on the left valve. Three cardinal teeth in each valve; centre one of the left valve, centre and posterior of right, bifid. Adductor scars and pallial line are distinct. Pallial sinus is short, U-shaped (I. irus).

Up to 25 mm long.

Yellowish white or fawn, usually darker about the umbones. Inner surfaces are white, with blue or chestnut patches. particularly posteriorly.

The animal is white, tinged with pink. The siphons are united for half of their length, and the valve of the lower is protruded beyond the fringe. The foot is rather small, compressed, and adapted for spinning a byssus(I. irus-animal).

In holes and crevices in rock, or in Laminaria holdfasts, from the lower shore into the shallow sublittoral.

In British Isles southern and western coasts only. Ranges south to the Mediterranean and West Africa.