Shell not very slender, biconical, with 5,75-6,25 nearly flat whorls (the initial ones often broken away). Sutures not always clearly discernible. Embryonic whorls not sculptured. Initial teleoconch whorls with three to four prominent spiral ribs and somewhat less conspicuous axial ribs. On the penultimate whorl the sculpture becomes more obsolete, and only (very) vague spirals, c. five above the aperture, are still discernible at the last whorl. Irregular, incised growth-lines are visible in well-preserved specimens. There are red-brown spiral lines all over the shell, c. 15 of which may be counted on the last whorl. The background colour of the shell is yellowish white, sometimes with a lighter zone around the periphery, where vague, widely spaced, vertically elongate, darker blotches may be observed in several specimens. Last whorl occupying c. 3/4 of the total height of the shell. Aperture narrow and elongate, with parallel lips; its height is slightly more than half the total height of the shell. Outer lip with up to eight plicae inside, the lowest ones being most delicate. Inner lip hardly calloused. Columella straight, with two rather prominent folds. The shells are 5.8-7.7 mm high and 3.1-4.0 mm broad.

Description. Shell of medium size for the genus, height 5.8–8 mm, width 3.1–3.85 mm. solid, fusiform, biconic, slender, H/W=2.064–2.133. Protoconch paucispiral, shiny, of 1.3–1.4 convex whorl, d 0.20–0.225 mm, Do 0.35–0.40 mm, DM 0.55–0.575 mm, h 0.45 mm, sculptured by sparse microgranules. Protoconch-teleoconch boundary well marked, flexuose, opisthocline. Teleoconch of da 5–5.3 barely convex whorls. Sculpture starting with 3 spiral cordlets crossed by 14–15 opisthocline axial ribs on the first whorl. Additional spiral cordlets between the adapical ones, and between the suprasutural one and the suture. Axial ribs present on all whorls (18 weak, orthocline or slightly prosocline axials on the penultimate whorl) except the last one, and producing nodules at the intersection with the spirals. Last whorl sculptured over the entire surface by 20–23 weak spiral cordlets, flat, equidistant, of equal size, as broad as the interspaces, of which 4–6 above the aperture. Two columellar folds, equal in size. Growth lines visible over the entire surface. Outer lip not sharp, thickened internally with 7 denticles, the second or third posterior more pronounced. Anal sinus very shallow. Siphonal canal short, and wide. Coloration, beige in the background with 16 chestnut-brown spiral stripes narrower than the spiral cordlets. Cordlets I, IV, VI, VIII, XI, XIV and XVI of the same background color. Often, whitish subsutural blotches in correspondence with the axials. Protoconch brown with lighter spiral suprasutural band. Aperture and columella lighter. Soft parts white translucid, with black eyes.

M. wilhelminae differs most clearly from M. olivoidea (Cantraine, 1835), the type species of Mitrolumna BDD, 1883, which has a quite similar protoconch (Cernohorsky, 1975: 232, fig. 59), by the obsolete sculpture of the last whorl and the conspicuous colour-pattern with spiral lines. The lectotype of Af. olivoidea, selected and figured by Cernohorsky (1975: 231, figs. 55, 56), has c. ten prominent spiral ribs on the last whorl above the aperture. M. wilhelminae differs from M. crenipicta Dautzenberg, 1889, by being relatively broader, with much less spiral ribs. The holotype of M. crenipicta appears to be rather faded now. However, the original description and figure also show the colour-pattern of M. crenipicta to be different from that of M. wilhelminae.

M. wilhelminae, known so far only from the area of Gibraltar and the Alboran Sea, is easily recognized by its shell inflated even in the first whorls, nearly keeled at the periphery, with weak spiral and axial sculpture, a light background color, with numerous fine spiral brown striae, often with bright coloration darker on the base, and blotches at the periphery. A peculiar deep water morph (corresponding to the western cline of M. karpathoensis sensu Mifsud 2001) is recognizable by its stouter outline, its usually more numerous spirals on the last whorl (34) and above the aperture (8), and its peculiar coloration with a beige-pinkish background with some darker spirals, and a darker peripheral band (corresponding to 3–4 spirals) with two spiral series of whitish rounded spots.

Distribution and habitat. Alboran Sea (Western Mediterranean). Recent. Empty shells from the intertidal down to 400 m depth in bioclastic sediments. Morph A, live specimens collected amidst photophylous algae (1–30 m), kelp (30–60 m), more rarely on hard coralligenous bottoms (60–250 m) (Peñas, Rolán, Luque, Templado, Moreno, Rubio, Salas, Sierra & Gofas (2006). Shells of morph B from deeper waters (80–180 m), on red coral bottoms (Gofas, Moreno & Salas 2011: 329).
The type is known from Getares, Bay of Algeciraz, Spain.