Shell. Except for its smaller size the shell of this species is at first sight nearly identical in appearance and proportions with that of M. alba. Closer examination shows a number of slight differences: the spire is usually straight (but the protoconch often a little bent on it) and looks straight-sided, but is cyrtoconoid (apical angle about 25° in young shells, 20° in older ones); the whorls are a little more tumid, the true sutures are more conspicuous; they are parallel to the false ones, the space between often white; they cut the spire axis at an angle c. 80°; the last whorl is less clearly angulated. The ornament is diagnostic — extremely fine but distinct spiral lines about 8 µm apart, seen only by reflected light, with growth lines as in M. alba, 2—3 times coarser than the spirals. Varices are common, with a simple, shallow crescentic shape.
The protoconch usually persists, its 4 whorls more tumid than those of the rest of the shell, and often less highly polished. It measures about 650—700 µm long and 350—450 µm across.
Aperture. Much as in M. alba, but less angular. The outer lip in apertural view continues the profile of the spire or may bulge outwards a little; in side view it shows a deeper anal sinus than alba. The canal at the base of the aperture is deeper and narrower than in that species. The columella has a narrower lip and joins the inner lip over the last whorl at an angle of 140—150°.
Colour. Milky white as in alba.
Size. 9x3 mm. Last whorl = 40—45% of total shell height; aperture = 25% of shell height, its breadth about 65—70% of its height. Animal. As in alba though the tentacles are shorter and stouter and the eyes a little more raised. In males the penis is thicker distally. Colour. White.

Geographical distribution. From Iberia to Norway.
Habitat. On soft bottoms of muddy sand and gravel from 14—100 m. Not common. Food. M. lubrica is an intermittent ectoparasite of the holothurian living on the soft bottoms on which it is found.