Shell small, robust, biconic, rather tall and narrow, with all the characteristic features of the subgenus. The colour of this species is very variable, from white to very dark brown base colour with a spiral band of alternating light brown flecks or slashes and white squares or blotches, which are most distinct on the body whorl. Sometimes the wavy spiral band is totally white. Lower part of body whorl darker than rest of shell. Protoconch small with about 1,5 white to light brown shiny whorls. Teleoconch whorls four to five, slightly rounded to rather flat. First two or three with axial and spiral sculpture usually forming distinct tubercles. Axial ribs, about twelve, usually very wide and distinct and sometimes overlapping on to the top of the body whorl. There are 5-6 spiral chords on the penultimate whorl, the first sub-sutural two rather wide, and about 20-22 narrower ones on the body whorl. Columella with two folds. Aperture narrow. Outer lip sharp, but thickened internally with internal small denticles. Canal short and open. Animal unknown. Size: 5-6 mm.
Material studied. All from Canary Islands.
Holotype, h= 5.5mm , w=2.5mm. El Hierro, La Restinga, 30-55 metres, iv 99, leg. W. Engl. BMNH 20000017

Juveniles of M. hierroensis n. sp could be confused with juveniles of M. cachiai n. sp. but the very distinct axial ribs, which sometimes overlap on to the body whorl of M. hierroensis n. sp easily distinguishes this species. Specimens of M. hierroensis n. sp. are not rare at the type locality but are usually found at depths of 30-50m or deeper. The species also differs from M. swinneni n. sp. by,
1. Its smaller size.
2. Its more robust shell and narrow profile.
3. Its more pronounced axial ribs.
4. Its much earlier development of the columellar pleats.
5. Its colour pattern.


M. hierroensis n. sp. differs from M. engli n. sp. by
1. Its more slender aspect.
2. The presence of axial ribbing extending only to the top part of the body whorl.
3. The number of more spiral chords on the penultimate and body whorls.
4. Its colour pattern, which always presents a darker shell base.
Some specimens of M. hierroensis n. sp. sometimes have a pupoid or elongated profile and may resemble juveniles of M. gofasi n. sp. However, they may be separated by the inferior number of spiral chords on the penultimate whorl and their very particular colour pattern.
Distribution: the Canary Islands, infralittoral.