he other, and some of its segments have coarsely pectinated spines.
Second cirrus has the five basal segments of its anterior ramus highly
protuberant, and paved with bristles, of which some are coarsely
pectinated; the basal segments of the posterior ramus are rather more
thickly clothed with bristles than are the posterior cirri, but
otherwise resemble them. The third cirrus, as already stated, is exactly
like the three posterior pairs; and this is a very unusual circumstance.
On the dorsal surfaces and sides of the pedicels of the posterior cirri,
there are some scattered, short, thick, minute spines.

_Caudal Appendages_, multi-articulate: in a medium-sized specimen, each
contained eight segments, which reached half-way up the upper segment of
the pedicel of the sixth cirrus. Lower segments flattened; the upper,
tapering, and cylindrical; all have their upper margins furnished with
stiff, little spines. In a young specimen (only .3 of an inch in length,
including the peduncle), the caudal appendage contained only four
segments, and the tip did not reach to the upper edge of the lower
segment of the pedicel of the sixth cirrus.

_Stomach_, without caeca.

_Generative System._--Vesiculae seminales not reflexed at their broad
ends; white, spotted with black. Testes, pear-shaped, borne on long
footstalks: penis covered with minute bristles, in little tufts arranged
in straight lines. The ovarian tubes fill up the peduncle to its base,
but do not surround the sack; they are of small diameter, and simply
branched. There is a very narrow ovigerous fraenum, with a straight edge,
lying on each side under the line of junction between the scutum and
upper latus.

_Affinities._--This species differs from all the others of the genus, in
the third cirrus resembling exactly the three posterior pairs. In most
of its characters--namely, in the symmetrical arrangement of the scales
on the peduncle, in the considerable size of the valves of the lower
whorl, in the general approximation of the valves, in the
multi-articulated caudal appendages, in the form of the outer maxillae,
in the prominent olfactory orifices, in the basal segments of the
anterior ramus alone of the second cirrus being paved with bristles,
there is more affinity to _P. cornucopia_, _P. elegans_, and _P.
polymerus_ than to _P. sertus_ and _P. spinosus_.

In the scuta and terga being articulated together, in the union of all
the valves by stiff membrane, in the