_Hab._--Imbedded in the skin of a sea-snake, taken off Borneo.
I am indebted to Dr. Gray for a single specimen of this supposed
species, but as it is very young and imperfect, wanting the opercular
valves and cirri, I do not choose to name it. The shell presents all
the usual characters of the genus; the rostrum, I may remark, being
pushed to the left side. The parietes are permeated by pores of
considerable size, which shows that the species is distinct from _P.
decorata_. On the inner basal surfaces of the walls, there are two or
three very distinct ridges on each side of the midrib; and this fact,
together with the size of the parietal pores, makes me suspect that it
is not an immature specimen of _P. bissexlobata_.