Shell in general like that of other Emarginula spp. but the ornament is much finer so that it seems rather smooth. White. Animal with 30 epipodial tentacles. Most radial ridges are composed of triplets of smaller ridges with the openings of pits in the intervening grooves appearing as white dots. Up to 30 mm long, 20 mm broad, 15 mm high. The animal is like E. fissura.

In general like that of fissura, the apex lying at about the same position relative to the posterior margin and highest point. Apical angle 80-95°. It bears a very large number of radiating ridges of which about 50 stand out more prominently. Many of these 50 are seen on closer examination to consist of three ridges closely apposed. Still smaller ridges lie in the intervening furrows. All are crossed by ribs of about the same dimensions as the larger ridges to produce a reticulation of the surface much finer than in either fissura or rosea and not always immediately visible. To the naked eye there also seems to be a micro-reticulation due to microscopically small ridges and ribs which appears as rows of white points on the surface of the shell: with higher magnification and cleaning these points prove to be rounded pits lying in rows in the furrows between ridges. The slit equals 1/5 of the distance from shell edge to apex; the slit band is marked by curved transverse lines. Internally the younger part of the shell shows folds corresponding to external ridges and the slit band shows as a thickened ridge.
Aperture oval, its lip smoother and thinner than in the other two species in conformity with the smaller size of the ridges.
Colour. Creamy white, sometimes flecked with brown, and showing rows of microscopic white spots in all the furrows. Littoral specimens often stained. Internally white and shiny.
Size up to 33 x 25 x 17 mm, commonly less. Height = 40-50% of length; breadth = 66% of length.
Animal like fissura but epipodial tentacles number about 30 on the foot with one behind right eye stalk.

Shell moderately heavy, large, reaching 27 mm. in length, conical, the height being only 40% to 50% of the length of the shell, although the elevated position of the apex gives to the shell the appearance of being quite high. Color opaque white or cream. Base broadly ovate. Anterior slope convex, posterior slope concave immediately below the apex, then extending downward in a straight line to the margin. Apical whorls one and one quarter in number, very small, close to but a little below the summit and at a point removed from the posterior end of the shell by about a third or a fourth of the length of the shell. Anal fasciole narrow and formed by very closely-set lamellae. Fissure small, one fifth to one sixth the length of the anterior slope. Sculpture compact, consisting of numerous, closely-set, slightly raised, radiating ribs. At the apex the primary ribs begin as fine threads, becoming stronger and broader as growth proceeds; lower down the shell, finer ribs are intercalated between them. Sometimes the primary ribs divide into three or four while still maintaining their identity as rounded bundles. Numerous concentric coarse and wavy cords cross the shell giving rise to nodules where they intersect the ribs. Since ribs and cords are closely set the shell has a granular appearance. Margin finely crenulated by the terminations of the ribs. Interior of shell smooth and opalescent, the anal fasciole being marked by a thick grooved callus. Length 21-27 mm; Width 13-20 mm; Height 10-11 mm

We have compared the types of E. magnifica with specimens of E. crassa from the British isles and have come to the same conclusion as Thiele, that no differences can be found which would justify their separation. Some European specimens are fasciculated while others remain regularly ribbed. The latter is the case with the Saint Croix specimens.

E. crassa is found under stones, rarely at L.W.S.T., more frequently sublittorally to 200 m. It has been recorded from south and west Ireland, the Clyde, and north to Shetland and Rockall. It is a northern species, commoner in Norway and Iceland.

Geographical distribution. This is a northern species and in Britain appears to be limited to W. coast of Scotland and the Northern Isles; also recorded from S.W. Ireland and Antrim. It occurs on Norwegian coasts from the Lofoten Islands S. to the Skagerrak.
Habitat. Found attached to the underside of stones and shells at LWST and extending sublittorally to 200m; more commonly sublittoral.

Pilsbry's types appear to be fossil shells and not dead recent specimens. Though described as a fossil form, E. crassa is found alive in European waters. So far no live material has been collected anywhere in the Western Atlantic.