Shell with 4-5 swollen whorls, apparently smooth and glossy and with green and purple refringence. Aperture prosocline and round; umbilicus large. Head with ventrolateral ridges along snout; foot with six epipodial tentacles on each side.
Though appearing smooth to the naked eye the shell surface bears growth lines and there are some spiral ridges and grooves on the base of the last whorl. The aperture is slightly oval with the outer Up arising at right angles to the surface of the last whorl; the umbilicus is sometimes partly covered by an out-turning of the columellar lip. Horn-coloured; some shells show a spiral brown band on the last whorl and perhaps on the penult as well: all show refringence. Up to 4-5 mm high, 5-6 mm broad; last whorl occupies about 90% of the shell height, the aperture about 45%.
The snout carries numerous papillae, and these also beset the sides of the cephalic and epipodial tentacles. An eye stalk lies lateral to each cephalic tentacle and a smooth-edged cephalic lappet lies on its medial side. A lobe, with smooth edge, lies on each side of the neck. The foot is rather narrow and has a longitudinal epipodial ridge on each side below which the tentacles arise, each with a pigmented tubercle ventral to its base. The operculum is multispiral and its outer surface is concave. Yellowish, with purple-brown lines on the sides of the foot, the snout and the neck lobes.

Shell. Glossy, translucent, smooth to the naked eye. In profile the general shape is a flattened, oblique oval; the spire is a depressed cone with apical angle 85-122°, commonly 105-110°. There are 4-5 whorls which are gently convex and dip noticeably to sharp sutures which lie at the periphery of older whorls. The shell is marked by very shallow growth lines which are prosocline; irregularity in their development (forming incipient ribs) causes the adapical edge of each whorl to be slightly sinuous and near the tip of the spire the minute ribs so formed may extend across the whorl abapically. Spiral ornament is absent except on the basal part of the body whorl where there are 15-20 low ridges and grooves; these extend into the conspicuous umbilicus, the mouth of which is slightly oval and partly occluded by the inner lip. The protoconch is smooth, of just over 1 whorl, with a pointed tip and measures about 160 µm across.
Aperture. Oval, its long axis inclined abapically outwards at an angle about 120° from the axis of the shell. It is prosocline, its plane cutting the vertical axis at an angle of 40°. The outer lip is thin adapically, tapering to the edge, but thickening abapically where it joins the columella. It meets the body whorl nearly at right angles. The columella is straight and the inner lip curves to meet the parietal region tangentially. The growth lines arc visible internally.
Colour. Horn-coloured to orange-red, with green or purple refringency. The apex is paler and the colour of live shells may be affected by viscera showing through. Many shells have a more or less distinct brown band on the body whorl (and sometimes the preceding one) placed about one-third of the distance between suture and periphery. Internally the same colour as externally with greater refringence.
Size. M. helicinus has been found in Greenland 9mm high x 11 mm broad, but it has rarely more than half these dimensions at the southern limits of its range. Body whorl = 90% of shell height, aperture = 45% of shell height.

M. helicinus is found on the lower parts of rocky shores and sublittorally to depths of a few metres, from Yorkshire northwards on the east and from North Wales, the Isle of Man and Dublin on the west; it also occurs on north and west Irish coasts, The species has a circumboreal range. The animals are to be found on Fucus, Laminaria, red weeds, in rock pools, and under stones, eating detritus and weeds. Breeding occurs in spring, eggs being laid on laminarian fronds. They are fertilized externally and about 100 are aggregated to form a mucous mass. There is no free larval stage and the young emerge as juveniles (Fretter, 1955).