Solitary. Body wide and ovoid, up to 8 cm long, obliquely attached by the base and left side and somewhat flattened. The test is rather soft and often thick, and may be translucent or opaque, but is usually rugosa and dark.
The siphons are prominent and ribbed; oral siphon near the anterior end and atrial siphon 1/3-1/2 of body length from it. There are from 30 to 75 branchial tentacles in two or three orders. The branchial sac is minutely plicated, with 60-70 longitudinal vessels. Stout, subclavate papillae occur at the junctions of the transverse vessels with the inner longitudinal vessels. There are no intermediate papillae. The stomach has longitudinal folds. Anterior part of the intestinal loop not bent dorsally. The ovary and testis are somewhat diffused as a yellowish mass over the stomach and intestine.
Attached to hard substrata; from shallow waters to at least 1200 metres; a northern species, extending from the Skagerak,
Along the Norwegian coast into the Arctic seas of Europe, Asia and Greenland. It is reported from the Faeroes and Shetlands.