_Shell with fine longitudinal ridges, ornamented in the lower part
by minute beads; parietes not porose: basal membrane equalling in
convexity the shell._
_Hab._--Pacific Ocean; Galapagos Archipelago; Lord Hood's Island, Low
Archipelago; Mus. Cuming, Darwin.
_General Appearance and Structure._--Shell oval, with the orifice
large. The walls are thick, and of less height from top to base than
in the last species. The surface (and this is the chief external
character) is marked by fine longitudinal ridges, each of which,
when closely examined, is found to be double (fig. 2 _b_), and near
the basis, where not abraded, to be ornamented with minute beads on
each side: these beads are largest on those ridges which border the
sutures. The parietes are not permeated by pores; on their inner
surfaces there are a few rather prominent but short ridges, instead
of the many finer ridges, as in _P. bissexlobata_. The sheath is of
great thickness; in young specimens layers scale off its summit, all
round the shell, as in Tubicinella; it is only slightly hollowed out
at its lower margin; it descends more than half-way down the walls.
The midribs, formed by the folded walls, are more prominent than in
the last species, and descend lower down; hence the basal membrane is
more convex than in the last species, for it projects downwards as
much as the shell projects upwards. In medium-sized specimens, the
midribs project inwards to a distance exceeding half the breadth of a
compartment. A less proportional length of the midrib, in an inward
direction, is formed by the inward folding of the wall, and a greater
length by the thickening of its inner longitudinal margin, than in
the last species. The basal edge of the inner end of the midrib is
smooth.
_Dimensions._--The average size of the numerous specimens which
I obtained at the Galapagos Islands was about .2 of an inch in
diameter, and I found none larger; but Mr. Cuming's specimens from
Lord Hood's Island are half an inch in diameter.
_Opercular Valves._--These closely resemble those of the former
species, with the exception that the carinal end of the growing or
under surface of the Tergum is much squarer.
_Cirri._--The only difference which I could perceive was, that the
rami of the first pair were not quite so unequal in length.
_Species Dubiæ._