The shell is fragile and inflated, consisting of 2 postlarval whorls of evenly convex outlines and a deep suture. The protoconch has at least 2.5 whorls (earlier part corroded). The adult whorls have no subsutural zone. Their sculpture consists of very regular spiral and sinuous axial lines. These are of equal strength on the first and second whorls; on older specimens however (4 whorls in the BIOGAS shell) the spirals become more and more dominant. The larval shell is brown with an obliquely reticulate sculpture in the lower 2/3 of each whorl and bent axial ribs in the upper third. In the older parts of the protoconch the axial ribs extend to the lower part of the whorl. The outer lip is broken in the type, but evenly convex in older specimens. The siphonal canal is very short. The columella and body whorl form a regular curve where they meet. The adult shell is light yellowish white. Dimensions: height of the shell 3.84 mm, breadth 2.20 mm; height of the aperture 2.40 mm, breadth 1.16 mm.
Remarks. We have selected as holotype a juvenile which had the best (although partly corroded) larval shell because of the importance of this character in the genus. A larger specimen with eroded larval shell, and therefore referable to the present species only with a little doubt, is 14.7 mm high and 7.5 mm broad (BIOGAS). The larval shell of sigmoidea has a bigger basal diameter than in L. blanchardi and lusitanica. The adult sculpture, although basically similar, has the axial ribs more sinuous in sigmoidea than in the two above mentioned species.