Shell very small, about 3.5 mm. in length, strongly compressed and high, the height being 70% to 85% of the length. Color white under a pale green periostracum. Base narrow ovate. Anterior slope strongly convex, posterior slope concave along its entire length. Apical whorls one and one half, the tip smooth and highly glossy, the rest rough. The apical whorls are turned backwards to such an extent that they project slightly beyond the posterior end. They lie on the right side of the shell. The anal fasciole runs from the fissure almost to the apex and it is provided with numerous incremental lamellae. Fissure at the summit of the shell, rounded at its upper end and narrowing down to a point. A groove extends from the fissure to the margin of the shell, along the mid-line of the anterior slope bordered by ribs a little stronger than the remaining ribs on the shell. Sculpture consists of numerous closely-set radiating ribs starting at the base of the apical whorls. There are no true cords, but the radiating ribs show all along their length elongated thickenings which are concentrically arranged. Between each two ribs there is a series of white punctules which alternate with these thickenings. Margin finely crenulated. Interior of shell glossy with the punctules of the outside showing through as white points. Internal septum long, extending down far beyond the lower end of the fissure. It is slightly convex and runs close to the anterior slope, thus giving rise to a funnel-like formation.

This species seems to be related to P. granulata but they differ, however, quite sharply. In profile the apical whorls of P. billsae are produced beyond the posterior margin, while in P. granulata they are located in front of the posterior margin. In P. billsae the posterior slope is strongly concave throughout its entire length; in P. granulata the posterior slope is straight below the apical whorls. Finally, the sculpture of P. billsae consists of very compact radiating ribs which show elongated thickenings along their courses while in P. granulata the radiating ribs are not so closely set and show granules rather than thickenings.

Types. Holotype, United States National Museum, no 333728, from off Sand Key, Florida, Kalis, station 104 in 92 fathoms. Paratypes, from off' Carysfort Reef, Florida, Albatross, station 2041 in 00 fathoms, in the United States National Museum; from off Key West, Florida, Eolis, station 333 in 110 fathoms, in the Museum of Comparative Zoology: and from Bahia de Coehinos, Atlantis, station 3332 in 175-225 fathoms, at the University of Habana.