The shell is small (maximum length 35 mm) and stoutly fusiform. The spire is moderately high, consisting of four or five angulate postnuclear whorls and a protoconch of undetermined nature. The suture is indistinct. The bodv whorl is moderately large and broad. The aperture is ovate to lenticular, with a broad, shallow anal sulcus. The outer apertural lip is barely erect and marginally crenulate, reflecting the sculptural pattern on the outer shell surface. At the base of the body is a moderately long, slender labial ceratus, grooved on its dorsal surface; just below the shoulder region, and for a short distance anteriorly, the inner surface of the outer lip bears five or six denticles. The columellar lip is adherent above, detached and weakly erect below. The siphonal canal is short, weakly recurved, and closed.
The body whorl bears six or seven strong, recurved, scabrously lamellate varices extending over the shoulder and attached, buttresslike, to the preceding whorl. Immediately below the persistent labial tooth there is a notchlike separation in the varix, the portion of the varix below this slightly offset dorsally (i.e. away from the direction of growth). Spiral sculpture consists of four primary cords on the body, alternating with secondaries and with a tertiary cord between a secondary and each primary. Secondary and tertiary cords are also found on the canal. Where the primary cords cross the varices, short, open, recurved, spinelike points are developed, all of these connected by a thick, ventrally fimbriate varical flange. Shell color is light brown. The aperture is white, tinged with deeper gray-blue within.

Puerto Potrero, Costa Rica (type locality of M. lugubris), southern Ecuador to Peru.