Description (after 53 shells from 19 localities). - The shell of the holotype has some¬what more than 2.5 protoconch whorls of the lecithotrophic larval type, and almost 8 teleoconch whorls. The colour is light yellow-brown; the shell is semitransparent, with a somewhat lighter protoconch. The first part of the protoconch has five or six equi¬distant spiral riblets, of which the last one to three are zigzag-shaped. After about half a whorl these become less obvious and disappear, except for the most abapical one which, contrary to the others, becomes more and more prominent, to such a degree that about the last 1.5 whorls of the protoconch are carinate (therefore the name M. carinapex). This keel lies below the middle of the whorl. Above the keel there are little flexuous axial ribs, rather distant from each other. Below the carina the whorl is smooth. Around the transition to the teleoconch the keel suddenly makes a dip in abapical direction and becomes the fourth spiral cord of the teleoconch whorls.
The teleoconch has rather convex whorls and a deep and wide suture. There are five spirals, the first four are nodulous, the last one is smooth. The first spiral and the fifth spiral of the preceding whorl are both close together, within the suture, the last spiral just adapical from the centre of the suture, the first spiral underneath. The siphonal canal is short and wide, situated in the direction of the longitudinal axis of the shell.

There is some variation in characters of the protoconch: (1) sometimes there are 3 instead of 2.5 whorls; (2) the spiral ribs on the first part of the protoconch are sometimes a littie granulated and/or the last one is a zigzag spiral thread; (3) the keeled spiral descends gradually (instead of abruptly with a sudden dip) to the basis of the whorl; (4) on some shells there are, below the carina, also a few very vague axial riblets.
The characters of the teleoconch are rather constant, except for the colour which varies from semitransparent white to orange. The greater part of the shells is yellow-white, with a protoconch of the same colour; only the darker specimens sometimes have the protoconch a little lighter. The largest shell has a length of 4.7 mm.
The teleoconch of M. carinapex has approximately the same characters as that of M. metaxae, although the first species never has flat whorls and its suture is somewhat wider. Nevertheless, the differences are insignificant and it is impossible to separate the specimens of both nominal species without a protoconch, when they are found together in one sample as happened frequently (the same applies to the teleoconch of M. hapax). On the other hand, the protoconchs of both species represent different larval types. The styloid protoconch of about 4 whorls of M. metaxae differs remarkably from that of M. carinapex.
M. abrupta, only known from the Azores, has a much broader protoconch (and first teleoconch whorl), which is 0.5 to 1.0 whorl shorter and lacks the conspicuous carina.
M. hapax differs from M. carinapex by its protoconch too (which has no keel) and by its continuous riblets over the total height of the whorls.

As far as known only dredged around the Cape Verde Islands in depths from 50 to 700 m.