Like M. groenlandicus but differs in having four swollen whorls with many fine spiral threads. Olive or grey with green or purple refringence. Animal with ventrolateral ridges on snout much reduced: five pairs of epipodial tentacles.
The shell is delicate and semitransparent, with deep sutures. There are prosocline growth lines present in addition to the spiral threads. The umbilicus is large and partially blocked by an out-turning of the inner lip. Up to 3 mm high. 4mm broad; last whorl occupies about 80% of shell height, aperture about half. The flesh is cream with some speckling of dark colour and there is a dark band along the edge of the mantle skirt.

Shell. Rather delicate, semitransparcnt, glossy. In general shape it is globose with a conical spire (apical angle 80-90°) of 4 convex whorls which meet at deep sutures at a level slightly below the periphery of the upper whorl (sutural angle 115-120°). Nucleus smooth, of 1,25 whorls, about 50 µm in diameter. The whorls are marked by many very fine growth lines, sometimes bunched and raised below the sutures to make ribs. More obvious is a series of slight spiral ridges, narrower than or equal to the intervening spaces, most clear on older whorls and the base, less obvious at the periphery of the body whorl. There are about 100 on the body whorl, 30 on the penultimate, 12 and 8 on the preceding two. They enter the umbilicus, which is large. In some shells they are alternately larger and smaller.
Aperture. Prosocline, making an angle of 120° with the axis of the helicocone. The outer lip arises nearly at right angles just below the periphery of the body whorl and sweeps in a smooth curve, almost circular, to where the inner lip ends as a glaze on the body whorl. The aperture has a thin edge everywhere. The inner lip partly blocks the umbilicus.
Colour. Olive, grey, cream or white with a green or purple refringence best seen on wet shells.
Size. Up to about 8x8 mm but those collected off Scotland were only about 3x4 mm.
Animal. Much like that of M. groenlandicus. The tip of the snout is not so extended laterally and the ridges along the sides of the snout are almost absent. The eye stalks are short; they separate from the tentacle only at the level of the eye. There are only 5 epipodial tentacles on each side of the foot.
Colour. Cream, with a few dark speckles and a dark band along the edge of the mantle skin.
Geographical distribution. Like other species of this genus, argentatus occurs primarily in Arctic seas and extends S., but less successfully than either helicinus or groenlandicus since it is both rarer and smaller in Britain, Norway and America. It reaches S. in Europe to W. Norwegian and Scottish coasts and in America to Massachusetts. It is not found in the Skagerrak, Kattegat or N. Sea.
Habitat. Sublittoral throughout it range; probably with the same mode of life as groenlandicus.
Food, breeding and growth. Unknown, but likely to be similar to the other species. Faeces form a rod similar to that of other species full of detrital material.

M. argentatus is an Arctic species reaching southwards on both sides of the Atlantic. It is sublittoral in its habitat and has been found at only a few places off Shetland and the west coast of Scotland.