t, broad, blunt.

_Mandibles_, with three main teeth, with either one or two smaller teeth
inserted between the first and second, making four or five altogether;
inferior angle rather narrow, pectinated with long and fine spines.

_Maxillae_, rather broad, with two long upper spines; beneath which there
is a very small prominence bearing a minute tuft of fine bristles;
beneath this, there are eleven pairs of rather long and strong spines;
and the inferior angle is formed by a rather broad, upraised, and
obliquely rounded prominence, bearing a broad tuft of fine spines.

_Outer Maxillae_, with the inner surface continuously clothed with short
spines; exteriorly there is a slight prominence with long hirsute
spines.

_Olfactory Orifices_ barely prominent.

_Cirri._--First pair placed near the second; the segments of the three
posterior pairs are slightly protuberant, and bear three or four pairs
of finely serrated spines; intermediate tufts long, the middle spines
being the longest; spines on the upper lateral edges long and strong;
dorsal tufts rather short. _First cirrus_, long, multiarticulate, having
fourteen or fifteen segments, whilst the sixth cirrus had nineteen
segments; rami unequal in length by about two segments; basal segments
protuberant brush-like. _Second_ and _third cirri_ with five basal
segments of both rami protuberant and brush-like; but the anterior rami
in both cirri are broader than the posterior rami. Spines on the
protuberant segments of both rami of both cirri, coarsely and doubly
pectinated.

_Caudal Appendages_ (Pl. X, fig. 19), minute, uniarticulate,
club-shaped, with the enlarged ends directed inwards, or towards each
other; summits sparingly clothed with very short spines.

_Penis_, small.

_Affinities._--This species makes a very close approach in the general
form and relative sizes of all the valves, and in the variability of the
number of the whorls, to _P. spinosus_; there is a still closer and more
important resemblance, in the inequality and manner of growth of the
calcareous scales on the peduncle. These species differ, in the colour
of the membrane covering the valves, and in the greater development of
both rostrum and sub-rostrum in _P. sertus_. The rostrum of the latter
is longer than half the length of the carina, and its inner surface is
more than twice as high as wide; and the sub-rostrum is twice as large
as any of the latera,--all points of difference from P. _spinos