mes more
on one side of the individual than on the other,) a nearly straight
line, running diagonally across the capitulum, of slight, quadrilateral
depressions, of a dirty greenish colour, with the edges blending away,
is either conspicuously developed, or can only just be discerned. These
marks increase in size from the umbones to the margins of the valves.
There are sometimes two or even three rows on the scuta. They are formed
by the retention of a portion of the chitine membrane, which is cast off
the rest of the surface; the margins of the valves are occasionally
notched slightly on the line of marks; there is no difference along this
line in the underlying corium. Specimens both with and without a barbed
carina are thus characterised. _Carina_; the interspace between the
carina and the scuta and terga is not wide. The carina exteriorly, is
either convex and smooth, or furnished with knobs or with extremely
sharp, long teeth (Pl. I, fig. 1 _b_); small specimens, with the
capitulum under half an inch in length, are generally most strongly
barbed.[26] Apex more or less acuminated; width and thickness variable;
sides strongly furrowed. Fork (fig. 1 _a_) generally less wide than the
widest upper part of the valve, with the two prongs diverging from each
other at less than a right angle; their sharpness and precise form
variable; rim between them reflexed (figs. 1 _a_ and _b_), making a
slight notch behind. _Peduncle_ smooth, wrinkled, length in proportion
to that of the capitulum varying, from barely equalling it, to six or
seven times as long. I have noticed a specimen including mature ova,
with a capitulum under half an inch long.

[26] Mr. W. Thompson found that 15 specimens, out of about 200,
attached to a vessel which came from New Orleans into Belfast,
had their carinas barbed.

_Filamentary Appendages_;--never more than two on each side, with
sometimes only one developed; of variable length; one seated on the
flank of the prosoma, under the first cirrus; the second close under the
basal articulation of this cirrus, on the posterior face of a slight
swelling: these appendages correspond with _g_ and _h_ in Fig. 4, Pl.
IX.

_Mouth._--Mandibles (Pl. IX, fig. 5), with, as usual, five teeth, all
pointing downwards. Maxillae (Pl. IX, fig. 9), with the lower step of
variable width compared to the two upper steps. _Cirri_; posterior cirri
with segments (fig. 26) bearing six pair of spines; intermediate fine