Shell oval-lenticular, white, fragile and not translucent. Protoconch not promi-nent, with little more than 1 whorl. The nucleus appears smooth, but magnification shows a slightly rough surface; the subsequent half whorl is smooth and on the last half whorl there are irregular nodular spiral lines that are only visible under high magnification. The axis of the protoconch is slightly tilted in relation to that of the teleoconch. The colour of the protoconch is white. Teleoconch with about 1,5 whorls, with rapid development; it is sculptured by fine axial ribs which are slightly curved, numbering about 18-25 on the first whorl, with wide spaces between them, but being almost imperceptible on the first half whorl. The spiral striae are small and numerous, and arc evident in the intervals between the ribs. There is a distinct angulation at the periphery. Aperture ovoid, with the outer lip sharp, only contacting the penultimate whorl for a short distance. An open umbilicus without any keel shows the previous whorls.

M. thompsoni may be differentiated from most of the Caribbean species by its peripheral angulation, being similar only to M. floridana. The differences with this last species are: M. thompsoni has the peripheral angulation less pronounced; its spiral striation is formed by threads more separated and differentiated; its protoconch is less tilted than that of M. floridana. Also the sculpture of its protoconch is different, having spiral irregular nodular threads on the last half whorl, instead of the smooth surface of M. floridana. As both species have direct larval development, the distance of both areas of distribution precludes contact between them.