Shell biconic, small, height 8 mm, width 3.1 mm (holotype). Basic colour, white, with yellow and sometimes brownish vertical or axial streaks, especially on the body whorl. Protoconch paucispiral, consisting of about 1 lA globular and glossy white whorls. Teleoconch with up to five whorls, (holotype) which are flat to only very slightly convex. The whorls, including the penultimate, are each sculptured with five to six, equally spaced, spiral chords. The thickness of these chords increases at each consecutive whorl. Sutures sometimes not very distinct, nearly at right angles to the shell axis. The body whorl takes up 75% of the total shell height and is sculptured with about 22-24 spiral chords. The primary subsutural spiral chord is occasionally wider than the rest. Growth lines opisthocline, dense, very conspicuous in the spaces between the spiral chords. Columella with two white folds, the upper one slightly larger. Outer lip with shallow subsutural sinus. There is a large tubercle on the inside of the outer lip, just below the sinus and about 7 smaller denticles just below it. Siphonal canal short and open. Animal unknown.
Material studied:
Holotype: Trawled between Dakhla and Cap Blanc, W. Sahara at 50-60 m deposited at The British Museum of Natural History N°1998017

The new species M. (M) denizi differs from all European, Mediterranean and NW African congeneric species by:
1. The absence of any axial ribs, especially on the first three teleoconch whorls.
2. Its constant white background colour and darker axial flammules.

A few South African species also lack the axial sculpture at the top three whorls. M. denizi n. sp. is similar to M. volva Sowerby, 1892 in lacking the axial sculpture at the top three whorls, however, M. volva has less spiral chords on the earlier and penultimate whorls and it is also of a smaller size. Albino specimens of M. olivoidea, M. mediterranea n. sp. and M. karpathoensis, although not recorded with certainty from this area, may be confused with this species, but the absence of any axial ribs on the primary teleoconch whorls easily separates the new species. The specimen figured by Knudsen (1956) as M. olivoidea is probably this species.