Shell biconic, rather smooth, with all the typical characteristic features of the subgenus. Protoconch of about 1,5 light brownish whorls. Teleoconch whorls flat to only slightly rounded, sutures only lightly marked. First three teleoconch whorls with light axial ribbing. There are 5-6 flat spiral chords on the penultimate whorl and about 20 on the body whorl, which also has a smooth median area. Shell colour greenish with many streaks, dashes and flammules of various shades of white, grey, brown and yellow, producing a rather beautiful marbled appearance. Internally the shell presents a white spiral band on a brown internal background colour, terminating near the sinus. There are two brown pleats on the columella the upper one slightly larger. Outer lip thin with an internal large tubercle, just below the sinus and a few smaller denticles below. Size 6-7 mm. The animal is entirely mottled white. Material studied; all from the Canary Islands.
Holotype; h = 7.5 mm w = 3 mm. Arguineguin, 5 m. viii 94 leg. F. Swinnen. BMNH 20000011
M. swinneni n. sp. species is very distinct from its colour pattern, its more flatter, smooth whorls and the wide, flat spiral chords on the penultimate whorl. It differs from M. canariensis n. sp. by its colour pattern, larger size and less spiral chords. A number of live specimens of M. swinneni n. sp. were collected by F. Boyer (pers. comm.) from the type locality. M. hierroensis n. sp. (see below) from El Hierro's, deeper water, 30-55 metres, is sometimes rather similarly coloured, but it has a shorter body whorl and clearly defined axial ribs producing a rather knobbed appearance to the shell. Distribution: at present known only from the Canary Islands.