Shell small for the genus, from 25 to 37 mm. Aperture roundly ovate. Columellar lip white, smooth, erect on 1/4 of its anterior part. Anal notch deep and small. Outer apertural lip erect and denticulate, briefly striate interiorly. Spire high, consisting of two glossy flat and rounded nuclear whorls and five to six slightly angular postnuclear whorls. Suture appressed. Body whorl bearing three more-or-less foliate varices. Ornamentation of the body whorl varices variable. Shoulder spine absent or very shallow for the holotype, which present only the last, anterior spine. Both paratypes with carinal spines followed by three others of which the second is almost invisible on the smallest paratype. Protoconch and first whorls are identical for both "forms". Other axial sculpture consisting of one large and one weak axial node; on holotype both axial nodes almost blended into a large single one. Spiral sculpture consisting of 9 or 10 squamous cords, usually with one intermediate squamous thread; sometimes two. Cords are slightly darker coloured on the largest paratype. Siphonal canal long and straight, narrowly open and slightly backward recurved at the tip; ornamented with three spines: two very near each other at the end of the canal and one situated medially, between the aperture and the first of the two last spines. The two last ones are strongly downward bent. Shell of a light brown color, aperture white. Dimensions : Holotype 37 x 15.8 mm

C. crosnieri is near two related species: C. artemis Radwin & D'Attilio, 1976 and C. rossiteri (Crosse, 1872). From both species it differs by its color, its larger aperture, its arrangement of spines on the siphonal canal and the very different protoconch: conical and slightly carinate for C. artemis and C. rossiteri, flat, slightly rounded and non-carinate for C. crosnieri. The operculum is almost identical with that of C. rossiteri. It must be noted that C. crosnieri sometimes lacks the shoulder spine in the same manner as C. rossiteri.

Tulear. South of Madagascar
This new species is named after Dr. Alain Crosnier, who collected this and many other interesting gastropods during his survey of the shrimp fishing grounds off Madagascar.