of one side of each segment covered with irregular rows
of long spines. Moreover, in the upper segments of these same cirri,
between each separate dorsal tuft, there is placed one or two long
bristles. The first cirrus appears to have had very broad segments, and
these are singular from the spines in the dorsal rows, being extremely
long. In some of the cirri, several of the basal segments are soldered
together.

_Caudal Appendages_, lost.

From the state of the specimen, it was quite impossible to ascertain
whether the individual here described was an hermaphrodite or female;
from the analogy of its nearest congener, _S. ornatum_, the latter is
the most probable; but the genus Ibla shows how the sexes may differ in
the most closely-allied forms.

_Affinities._--From the hollows on the under sides of the scuta, for the
lodgment of the males; from the umbones of the scuta and of the carina
being situated on the apices of these valves; and from all the
characters of the mouth, _S. rutilum_ is much more closely allied to _S.
ornatum_ than to any other species.

MALE, OR COMPLEMENTAL MALE.

In the concavity or hollow above the depression for the adductor muscle
(Pl. VI, fig. 2 _a'_), I found males, but in so extremely decayed a
condition, that they could hardly be examined. On one side, however, I
distinctly saw the larval prehensile antennae, with pointed, hoof-like
discs; and part of the thorax, with its small limbs and long spines, as
in _S. vulgare_ or _S. ornatum_. I also saw clearly the eye. The four
calcified beads or rudimentary valves, I believe, were present; but in
removing the specimen, the whole fell to pieces and was lost. The outer
integument was covered with rather thick, very minute bristles, each
about, 2/10,000th of an inch in length, and therefore only half the
length of those on the complemental males of _S. vulgare_. The cavities
for the males are not formed, as in _S. ornatum_, by the thickening of
the internal surface of the valve round a defined space, but by the
scutum being externally convex and internally concave down the middle,
hollows being thus produced both for the lodgment of the males and for
the attachment of the adductor muscle. These hollows are separated from
each other by a slight transverse ridge. I do not know at which point of
the margin of the valve, the orifice of the male is situated, but I
presume close under the apex. In this species, as in _S. ornatum_, there
can be n