Shell of medium size for the genus, height 5.4–7.9 mm, width 2.7–3.9 mm, solid, fusiform, biconic, slender, H/W=1.910–2.272. Protoconch paucispiral, shiny, of 1.25–1.5 convex whorl, d 0.175–0.25 mm, Do 0.30–0.45 mm, DM 0.45–0.60 mm, h 0.40–0.55 mm, covered by numerous microgranules, sometimes aligned and often welded to each other particularly on the first half whorl; slightly flexuose axial lines on the last 0.25 whorls. Protoconch-teleoconch boundary well marked, flexuose, opisthocline. Teleoconch of 4.5–5.2 barely convex whorls. Sculpture starting with 3 spiral cordlets crossed by 11–13 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 (20 on the penultimate) except the last one, and producing nodules at the intersection with the spirals. Last whorl sculptured subsuturally by 30–40 spiral cordlets, of which 3–4 above the aperture and twice as broad as the interspaces, and 10–15 on the base, equidistant, of variable size; periphery smooth or very weakly striated. Two columellar folds, the posterior larger. Orthocline growth lines visible over the entire surface of the last whorl. Outer lip sharp, thickened internally with 10–11 denticles, the second or third posterior more prominent. Anal sinus shallow. Siphonal canal short, and wide. Coloration monochrome, dark brown, with a lighter (almost white) band above and below the suture, with darker blotches on the adapical part. Often, 4–5 brown bands in correspondence of some spiral cordlets are visible. Threefour spiral cordlets on the base with alternate white and brown blotches. Soft parts white hyaline, with sparse milky white spots.

M. karpathoensis has less spirals than M. olivoidea (Nslw (aa): 8–11 and Nslw: 34–39 v. Nslw (aa): 3–4 and Nslw: 18–25 in M. karpathoensis), and lives in shallower habitat. M. wilhelminae has a more inflated outline in the juveniles (almost keeled), and a lighter coloration with darker spiral striae and alternate dark and light blotches.

Central and eastern Mediterranean Sea.