Shell helicoid with large last whorl and moderately high spire; ornament variable but commonly there are well-developed spiral ridges, though in some shells these are limited to the periphery of the last whorl. Aperture prosocline; umbilicus large. Seven pairs of epipodial tentacles.
There are 5-6 whorls which are rather tumid. The ornament is variable but usually there are 21-25 spiral ridges on the last whorl, 9-10 between the suture and the periphery and 12-15 between the periphery and the umbilicus; the furrows between ridges tend to be broader in the adapical half of the whorl and the most basal ridge forms a sharp boundary to the umbilicus. There are 5-7 ridges on the penult whorl, 4-5 on the others. The aperture is quadrangular. Cream, orange or pink, without refringence except in the throat. Up to 7mm high, 5mm broad; last whorl occupies about 85% of shell height, aperture about 55%.
The body of this animal has essentially the same structure as that of M. helicinus except in having an additional pair of epipodial tentacles; four pairs of the tentacles arise alongside the operculum. The flesh is cream and there are dark lines along the sides of the foot, on the tentacles and on the mantle skirt. The sense organs related to the epipodial tentacles are black.

M. groenlandicus is a northern species at its southernmost limits in the British Isles, where it occurs only off Scottish coasts. It is rare, living from L.W.S.T. to depths of 75 m, on weeds and under stones, feeding on the weeds or on detritus. The only observations on its breeding relate to East Greenland (Thorson, 1935) where spawn like that of M. helicinus was found fastened to weeds. The young emerged as miniature adults.