Shell elongate, globose, sub-rhomboidal, inequilateral, very convex, with a rounded umbo sets at the anterior end. Ventral margin almost straight, rounded posteriorly. The valves surface has a sculpture divided into three parts: the anterior one with 15-20 radial striae, the central one smooth practically, and the posterior one still with 25-30 radial striae finer than the anterior ones. The whole surface has thin growth stages. Smooth in the umbonal area. The shell background colour pattern is more or less light green, with more or less big brown spots marble-as.
Periostracum thin and bright, greenish in colour. The juvenile shells arc rounder in form and they have a less protuberant umbo.
Average measures are around one centimetre.

Habitat: this shell lives in low depth. We can often find it at the Laminaria base together with Ascidia mentula and Ascidiella aspera Tunicates (along Norway coasts with Ciona intestinalis too), or under the rocks. Distribution: common all over the Mediterranean.