Shell sinistral, tower-shaped, with 15 flat-sides whorls; sutures are shallow. Sculpture of spiral striae and transverse costae producing spiral rows of tubercles. Last whorl with three rows of tubercles above two or three nodular spiral ridges; tubercles more rectangular as they approach outer lip on abapertular side of the last whorl. Penultimate and two or three whorls above with three rows of tubercles, the central row being smaller; upper whorls with two rows. Aperture is angular, with small, narrow, almost tubular siphonal canal and a slight anal sinus. Outer lip is crenulate and thin; inner lip is reflected over columella; no umbilicus (M. adversa-drawing).
Up to 7 x 2 mm.
Chestnut-brown; tubercles, apex, and edge of outer lip are often lighter.
Snout is short; cephalic tentacles are long, slender with eyes at their base. Mantle is drawn into a siphonal spout on the right. Foot with a propodial flap above and behind anterior margin; posterior pedal gland opens into a median groove on sole; operculum is multispiral.
On, or embedded in, sponges, under stones, or on weeds from LWST to 100 m.
From Biscay to Norway; absent from southern North Sea and eastern Channel basin (Distr. M. adversa).