he fourth is pectinated and placed very close to the
inferior angle, which is produced into a long thin tooth. Maxillae
unknown.

_Cirri._--First pair lost. The arrangement of the spines on all is most
abnormal, Pl. X, fig. 29: dorsal tuft long, arranged in a transverse
line and seated in a deep notch; in the sixth cirrus, the spines on the
lower segments are fine, those on the upper segments are thick and
claw-like, mingled with some fine spines; in the four anterior cirri the
spines of the dorsal tufts are even thicker and more claw-like. On the
anterior faces, also, of all the segments the spines form a single row;
they are shorter than those composing the dorsal tuft; hence the spines
on each segment are arranged in a circle, interrupted widely on the two
sides: this arrangement is common to all five posterior cirri. Second
cirrus, with the _anterior_ ramus one third longer and thinner than the
posterior ramus (this is the reverse of the usual arrangement); this
longer ramus equals in length the sixth cirrus. Third cirrus, with the
anterior ramus considerably longer than the posterior ramus; in the
three posterior pair of cirri, also, the anterior rami are a little
longer than the posterior: except in length, there is little difference
of any kind between the five posterior pair of cirri. Pedicels of the
cirri long; rami rather short; segments elongated, not protuberant.

_Caudal Appendages_ nearly as long as the pedicels of the sixth cirrus,
thickly clothed with very fine bristles, like a camel's-hair pencil
brush.

_Affinities._--In the structure of the carina, and more especially of
the scuta, there is a strong affinity between the present and following
species; for we shall immediately see that in _P. eburnea_ there is
evidence of the scuta being composed of two segments fused together; and
the larger segment is furnished with an internal oblique, strong, basal
rim. To this same species there is an evident affinity in the form of
the mandibles and of the caudal appendages, and in the anterior rami of
the cirri being longer than the posterior rami. Notwithstanding these
points of affinity, I consider that _P. fissa_ is more closely related
in its whole organisation, as more particularly shown in the arrangement
of the spines on the cirri and in the presence of terga, to _P. crassa_
than to _P. eburnea_. Although in Dichelaspis, the scuta are invariably
composed of two almost separate segments, yet _P. fissa_ sh