Shell small, rather full, with a brilliantly polished olivaceous epidermis, and faint sculpture of incremental lines; umbones not prominent; anterior end rounded ; base nearly straight; posterior extremity bluntly truncate with a double flexure, caused by two well-marked ridges extending from the beaks to the extremity of the shell; lunule and escutcheon linear or none; ligament external, short, black; hinge line straight behind the beaks, descending slightly in front of them, with nineteen anterior and twenty-five posterior, small, short V-shaped teeth, the two series separated by a short edentulous space; interior polished, slightly iridescent; muscular scars rather large, faint; the pallial line obscure, with a large rounded sinus; margins simple, smooth. Longitude of shell 15.5; altitude, 0; diameter, 6mm.

This species is most like M. arruana Smith from the Arru Islands, but that species has the upper posterior corner beveled off and no longer angular, which makes a marked difference in the outline of the shell. M. obtusa, which has somewhat the same form, wants the marked furrows of the posterior end of this species.

Off the coast of Ecuador