Conical shell up to 20mm in length (var. pellucidum tends to be smaller). Shell is brown or olive in colour and can either be tranlucent (var. pellucidum) or opaque (var. laevis). The surface of the shell is smooth with very fine radiating ridges. Conical shaped Var. laevis tends to have a thinker shell and more pronounced conical shape.
- Blue rays on apex which often extend down to margin
Typically found on Laminaria but may also occur on fucoids. There are two varieties: var. pellucidum which occurs on the fronds and stipe of the Laminaria and var. laevis which occupies the holdfast. Breeds mainly in winter and spring. Fertilisation is external and the larvae initially settle on fucoids, migrating to Laminaria when they are about 4mm in length.
Found on all Atlantic coasts from Portugal to the north of Norway.