Description. Shell tall skeneiform, large for the group, smooth, opaque, colourless. The protoconch is not known. The teleoconch whorls are more than 2.5 in number, lack visible sculpture, but are neither polished nor glossy. The parietal callus is well developed in the adult, thin in the subadult specimen. The suture is deep; the whorls are tangentially cemented to the preceding whorl. The cross section of the whorls is round and not indented by the preceding whorl. The peristome is not thickened, prosocline, more tangential than radial. The umbilicus is wide and deep, narrower in subadults. Dimensions, diameter of holotype 2.3 mm, maximum size 2.5 mm (2 specimens).
Operculum moderately thin, yellowish, multispiral with central nucleus and short growth /one.
Radula c 35 - 3 -1 - 3 - c 35. The central tooth has a large, triangular, finely serrate cutting plate and broadly expanded lateral supports. The laterals have sharp, finely serrate cutting plates, broader in the outermost one. The marginals are rather broad and flat, tightly appressed to each other, and apically truncated. The apical part is denticulated; in the more central teeth, the inner edge has an apical major cusp, followed by half a dozen smaller cusps along the inner vertical margin. The apical margin in these teeth is oblique, slanting outwards, and has about eight denticles. Towards the outermost marginals this asymmetry disappears, replaced by more uniformly oar-shaped, weakly denticulate teeth.

The shell of P. thorvaldssoni is very similar to that of P. valvatoides, although there may be differences in the protoconch, which remains unknown in P. thorvaldssoni. The type species, however, has a thick periosiracum (hardly noticeable in thorvaldssoni). and longer and more finger-like denticles on the marginal radular teeth compared with thorvaldssoni.