Description. — See also Hemphill (1894). Shell conic elongate, with about eight whorls. Rather fragile. Protoconch smooth, with the characteristic shape of the genus, with approximately one whorl, a nucleus with a diameter of about 40 µm, and a protoconch of about 180 to 200 µm. The teleoconch begins with a little more than half a whorl which is evenly convex and has fine spiral striae. After 5/8 of a whorl a depression is formed below the suture which turns into a shallow canal that then becomes broader and forms a small shelf which separates the upper and lower parts of the whorls. The upper part represents between 20 and 25% of the total height of the whorl and is almost smoooth, but on the subsutural region short opisthocyrt growth lines can be observed crossed underneath by scarcely noticeable spiral lines. The lower part is separated from the upper part by a small shelf or step, and has between 10 and 15 irregular and variable spiral furrows at variable intervals. Down to the base of the last whorl up to 30 of these furrows can be seen. The furrows are crossed by prosocyrt curved growth lines. Towards the base the furrows are finer. Aperture rounded, umbilicus narrow or very narrow if the columella is a little curved outward. Dimensions: the larger shells reach up to 4.0 mm in height.
Distribution. — California (USA), Lower California, Gulf of California (Sea of Cortez) to south Mexico, Between almost 33°N and 19°N. Living specimens were collected between 9 and 27 m.
Remarks. — This species is relatively variable, but the abundant material examined indicates a single species.
The most similar species are M. evermanni (Baker, Hanna & Strong, 1928) and M. galapagensis spec. nov.