Dimensions of adult shell 1,5x0,9 mm, Protoconch 1 1/4 whorl, apparently smooth or with a vague indication of spiral sculpture, demarcated from teleoconch by a faint line. Teleoconch with 3 rather convex whorls. Sculpture very faint on spire, with a thickened line running along the suture; anterior part of body whorl with 7-8 incised spiral striae, and a small umbilical chink. Aperture not thickened, smooth inside. Shell colour tawny, translucent, with a series of subquadrate patches along the suture and vaguely articulated lines along the spirals on anterior part.
Head with bilobed snout, jaws and yellow buccal mass seen by transparency. Tentacles rather thick, parallel sided, with a solid axial white line. There are two yellow patches to the inner side of the eyes, and an area vaguely tinged with grey over the head. Foot slender, with anterior pedal gland not very distinct, and a yellowish mass beneath operculum. There is a cluster of three very short metapodial tentacles. Neck lobes are not developed, and opercular lobes are vaguely tinged with grey. There are two tiny pallial tentacles; the mantle shows, by transparency, some spirally arranged yellow and black specks.

This species is not Setia quisquiliarum (Watson, 1886) despite some similarity with Watson's illustration. The holotype of S. quisquiliarum has a clearly thickened outer lip and the apex shows remains of a brown stain, characters which point to affinity with a Rissoa or Setia sp. The spiral sculpture of Alvania postrema is of very finely incised spiral grooves, whereas that seen on the holotype of S. quisquiliarum is of very faint ridges, Alvania (Crisilla) picta (Jeffreys, 1867) has less convex whorls and a stronger spiral sculpture (Verduin, 1984).
A small Alvania (Crisilla) ohserved at Ceuta, strait of Gibraltar, is extremely similar to this species in all respects, shell characters and external morphology of the animal. It differs in having the tentacles with solid yellow axial line, and metapodial tentacle rather triangular and lobate.