Shell elongated, slender and conical, reaching 5.7 mm in length and 1.1 mm in width. Protoconch narrow, with truncated apex, three whorls, outline slightly angled; first whorl with tapered apex, sculptured by strong, short pustules of irregular shape and size; remaining whorls with numerous axial ribs discontinuous at middle of the whorl, with summits partially covering the suture. Teleoconch with up to nine whorls and weakly developed shoulder; ~21 axial ribs on fourth whorl, crossed by four spiral cords, the subsutural one being less prominent; a very small cord appears just above the shallow suture; axial sculpture very weak, sometimes disappearing even in the first whorls, and there are very small nodules or only slight elevations on the intersections with the spiral cords; smooth subperipheral cord on last whorl, base short and concave; aperture almost round, with an open siphonal canal; outer lip thin; columellar lip sinusoidal. Shell entirely white, sometimes golden-white.

The main difference between M. rugulosa and M. prompta is in the protoconch, since M. prompta has a narrower protoconch, with a unique distribution of axial ribs. The weaker axial sculpture and the less shouldered shell of M. prompta are other important aspects to distinguish the two species.

Geographic distribution. Bermuda (type locality); Brazil: Bahia, Espírito Santo and Vitória-Trindade Chain (this study).