Shell very small, conical, white, and translucent. Protoconch large and subulate, smooth, conical, of 5 whorls, the apex acuminate, the penultimate whorl swollen, adult sculpture beginning abruptly. Mature whorls 3,5, rounded, sutures deep. Sculpture of a few large, rounded, oblique axial ribs, curving in to the sutures, 13 on the body whorl, 11 on the penultimate whorl. Aperture semicircular, extended laterally and anteriorly, very oblique, the anterior margin thrust forward. Varix wide and prominent, very wide over the columella, where it is indented by the termination of the ribs. Length 2,3 mm.
Locality.—Christmas I., holotype and 23 paratypes.
Remarks.—This and the preceding 3 species are all closely related, and from the data available afford an excellent example of geographical differentiation, oceanica from the Indian Ocean, darwinensis from the Arafura Sea, excelsis from Torres Strait, and subulata from the Barrier Reef, southern Queensland. P. oceanica is the smallest of all, it has fewer whorls, and the peculiar protoconch is quite distinctive. The only other example among the rissoinids of a nearly similar protoconch is that of Rissoina ephamella Watson from 40 fm off Honolulu (Watson 1886, Plate 46, Fig. 6), but this differs in other characters, particularly the possession of a basal fold. In Watson's work, though the description of R. ephamella is indexed as occurring on p. 617, it has evidently been inadvertently omitted from the text.