Shell small, slender, white, opaque, with two nuclear smooth whorls and seven subsequent whorls; nucleus very small, blunt; subsequent whorls moderately rounded; suture distinct, with nine coronating points separated by deep and conspicuous pits; the ribs corresponding to the points are wide and thick but obsolete on the periphery, appearing again near the margin of the basal disk, where they are separated by shallow but distinct depressions; surface microscopically sculptured by sharp revolving lines reticulated by somewhat fainter incremental lines; basal disk slightly flattened, its periphery slightly angulating the whorl; aperture nearly circular, surrounded by a thick, slightly reflected peritreme. Height of shell, 11; of last whorl, 5; of peritreme, 2.7; maximum diameter, 4.0 mm. Type. Cat. No. 59336, U.S.N.M. Acapulco, Mexico; W. H. Dall. This is easily distinguished from the preceding by the smaller number and greater strength of the ribs. O. mazatlanica is more like O. hotessieriana Orbigny, of the West Indies than any other west American species known to me.