Very peculiar and well known species. Its form and sculpture are unmistakable. Little slim in form, with convex whorls and broad sutur. The sculpture is made by a few sinuous axial ribs crossed by several spiral little ridges. Also the lip is peculiar: rather thick, rounded, smooth internally, with canaliculi, crossed by prominent little ridges. At the base of the last whorl it is present a carina making a waste canal. The average measures are 2-2.5 mm.
Michaud (1832) set up the subspecies exigua, to point out the Mediterranean specimens, which are smaller than the Atlantic ones usually, but there are no differences in the sculpture and/or in the protoconch so to justify the use of this taxon.

Shell small, semitransparent, with long spire; whorls tumid, sutures deep. Whorls crossed by prominent sinuous costae, with numerous slight spiral ridges between and over them. Aperture small, obliquely set, with thick edge (double peristome). Umbilical groove bounded above by prominent keel. Animal has right and left pallial tentacles and a 3-pointed metapodial one.
The shell has six whorls with a blunt tip. The most conspicuous ornament is the series of flexuous costae, mainly opisthocline, and narrower than the intervening spaces. There are ten on each whorl except the oldest. The spiral ridges are low and strap-shaped, usually becoming eroded from the costal summits; the last whorl has a strong spiral keel below the ends of the costae, separated from the apertural edge by a deep umbilical groove. There are 16-18 spiral ridges on the last whorl above the keel, 14-15 on the penult, reducing up the spire. The aperture is bounded by a thick labial varix. White. Up to 3.5 mm high, 2 mm broad; last whorl occupies 60-65% of shell height, aperture 40%.
The snout is rather deeply cleft at its tip, the tentacles long and slightly flat, each with a basal eye. The left pallial tentacle is the more obvious. The foot narrows a little in the middle and the sole has a median groove to which a gland opens. The median part of the metapodial tentacle is long. Yellowish.
M. crassa is found between the Mediterranean and Norway. It is widespread in the British Isles, occurring occasionally under stones or with weeds at L.W.S.T. and on sand to depths of 50 m. It is said (Pelseneer, 1935; Vahl, 1971) to eat coralline algae, but may be grazing growths and detritus on their surface rather than the algae themselves. The life history includes a free veliger larval stage.

Distribution: very common all over the Mediterranean. Habitat: it lives in low depths in a detrital bottom.