Diameter of base up to 20 mm, height in full extension to about 60 mm.
Base a little wider than column, firmly adherent. Column fairly tall in extension, distinctly divided into scapus and scapulus, the former with small conical or rounded tubercles and strongly adherent periderm, the latter often absent from the limbus region. Periderm thin but tough, horny in texture and usually deeply wrinkled, often with algae or encrusting animals such as bryozoans or serpulid worms growing on it.
Tentacles: Moderate in length, usually irregularly arranged, up to about 120.
Colouration: Scapus beneath periderm yellowish or olive green with dark brown tubercles. Scapulus translucent, flesh-coloured or greyish, darker above, with irregular streaks of dull purple on the older endocoels, these running on to the upper part of the scapus. Disc and tentacles dusky grey, red, brown or yellowish, disc with a pale area around the mouth. Usually the older radii are darker-purplish or reddish-and the mesenteric insertions pale, producing an ill-defined pattern of alternate dark and light radial markings. Tentacles usually with two whitish bands.
Asexual reproduction by basal laceration is habitual and frequent.
Nematocysts of acontia: p-mastigophores 30-42 x 3.0-4.0 öm, b-mastigophores 33-44 x 3.4-5.0 öm.
Occurs on rocky coasts from about LWST to 15 m depth, usually where wave turbulence is strong. Attached to open rock faces or in pools and clefts.
In Britain this species is recorded from the type-locality in north-east Scotland, where it still occurs, and has recently been discovered in several localities in north-west Ireland, where it is abundant. Elsewhere it is known from Iceland and Norway.