much shorter
than the outer; and this seems to be connected with the flatness of the
whole animal, and the consequent little power of divergence in the rami
of the cirri. The first cirrus is rather short, with the rami unequal in
length by about two segments: the anterior ramus is shorter and thicker
than the other: segments numerous, each clothed with several rows of
bristles. The second cirrus has the anterior ramus thicker and more
thickly clothed with spines than the posterior ramus; this latter is
rather more thickly clothed with spines than are the three posterior
cirri; the third cirrus is in all these respects characterised like the
second cirrus, but in a lesser degree. The pedicels of the second and
third cirri are thickly and irregularly clothed with spines; in the
three posterior pairs, the spines are placed in two regular rows, with
some minute intermediate spines.

_Caudal Appendages_ (Pl. IV, fig. 8 _a'_, _f_), multiarticulate, thin,
tapering, in one specimen equalling, in another just exceeding, in
length the pedicels of the sixth cirrus. In the latter specimen there
were thirteen segments, of which the basal segments were broader and
shorter than the upper; these latter are slightly constricted round the
middle, so that they resemble, in a small degree, an hour-glass. Their
upper margins are surrounded by rings of bristles; the terminal segment
being surmounted by one or two very fine bristles much longer than the
others. The two appendages are closely approximate; each arises from a
narrow elongated slip, attached to the side of the pedicel of the sixth
cirrus.

_Nervous system._--I examined the upper part of the nervous chord, in
order to ascertain whether the infra-oesophagean ganglion, which is of a
globulo-oblong shape, was far separated from the second ganglion; and
this I found to be the case, in accordance with the distance of the
first cirrus from the second. I may here remark, that in _S.
quadrivalvis_ I discovered the eye, which, though in all probability
really double, appeared to be single; it was situated near to the
supra-oesophageal ganglion; and this ganglion was situated near to the
adductor scutorum muscle, and at a considerable distance from the
labrum. The aperture leading into the acoustic (?) sack, is situated
much lower down than is usual (Pl. IV, fig. 8 _a'_), namely, at the
length of the pedicel of the first cirrus beneath its basal
articulation.

_Generative system._--The