_Shell reddish: moveable scutum, with three strongly prominent
longitudinal ridges, besides the articular ridges: fixed scutum larger
than the carina, with no distinct adductor plate._
_Hab._--West Indies, Mus. Brit.; attached to a Gorgonia.
This species differs considerably from all the others in the genus.
The shell is brownish-red, tinted yellow: it is not at all depressed
like the former species, but the walls are almost perpendicular or
even overhang their bases, and the summit of the shell consequently
is broad. This form may be in part, but only in part, due to the
attachment on the thin branches of the Gorgonia. The umbones of the
compartments are remarkably prominent and sharp. Although the parietes
are nearly smooth, yet from being so steep, they are little seen, and
owing to the very prominent but rounded ribs by which the compartments
and opercular valves are articulated together, the whole shell has a
strongly ribbed appearance. The diameter of the largest specimen was .2
of an inch.
The _rostrum_ (A, fig. 5) is patelliformed, with the umbo of growth
sub-central, but rather above the middle point; hence this valve,
differently from the carina, and differently from the rostrum of the
other species, grows not only at its basal margin, and on both sides
where opposed to the carina and fixed scutum, but also along its upper
margin where opposed to the basal edges of the moveable scutum and
tergum: owing to the perpendicularity of this valve, the upper part
forms a ledge almost parallel to the orifice of the shell. The carina
(B) is of unusually small size, being about only half the size of the
rostrum, and scarcely exceeding in size the fixed tergum. The _fixed
scutum_ (S′) is large, larger even than the carina; it is oblong, and
its shape is more simple than in the other species; this is chiefly
owing to the rostrum articulating with the whole of that margin (_b_)
which answers to the basal margin of the moveable valve; whereas
in the other species (fig. 1 _b_) it curls beyond this margin, and
articulates with the very protuberant, so-called, parietal portion of
the valve. Three or four rounded prominent longitudinal ribs, exactly
like the homologous ribs on the moveable scutum, run from the apex
of the fixed scutum to the basal margin, and their extremities form
the teeth by which it articulates, as just stated, with the rostrum.
Its upper articular ridge (′) is more prominent, and placed much
lower down in the suture between it and the fixed tergum, than in
the foregoing species. The ledge (_o_) by which the orifice is kept
neatly closed, is here more distinct than in _V. Strömia_: this ledge
is necessary, as well as in the case of the fixed tergum, owing to
the altered shape of the summits of the moveable scutum and tergum,
due to their corrosion and to their coming to project freely. But the
most remarkable character of the fixed scutum is, that on the under
side there is no great adductor plate, but a rounded hollow with its
lower edge only slightly prominent; the absence of the adductor plate,
which is present in all the other species of the genus, is no doubt due
to the under side of this valve being inclined even outwards, and so
standing in some degree opposed to the moveable valve; thus affording
on its under surface a place for the attachment of the lower end of
the adductor scutorum muscle; whereas in the other species this muscle
could not possibly have been attached, without the aid of an adductor
plate, to the under side of the much depressed and sloping fixed
valve. The _fixed tergum_ (T′) is a little more simple in form than
the corresponding valve in the other species; the two arms, answering
to the occludent and carinal margins of the moveable tergum, are more
nearly equal in length: the internal transverse ledge, separating these
rims or margins from the parietal portion of the valve, is but little
developed.
All four valves forming the shell are remarkable from having, when
full-grown, but not whilst young, their basal edges abruptly inflected
inwards, thus forming a ledge all round the basal membrane, as in
_Chthamalus intertextus_ and _Hembeli_.
_Moveable Scutum._--This is slightly larger in proportion to the
tergum than in the foregoing species: it is chiefly remarkable from
the presence of three prominent longitudinal ridges on the main part
of the valve, like the two articular ridges on the tergal margin; of
these latter, the lower one extends down to about the middle of the
tergal margin. The _moveable tergum_ is rhomboidal, with the whole
carinal portion marked only by lines of growth: it is only remarkable
by the upper of the three articular ridges on the scutal margin being
unusually distinct from the occludent margin.
With respect to the animal's body, its several peculiarities have
already been pointed out under the genus. The _labrum_ is decidedly
bullate, triangular in section, with a row of minute bead-like teeth on
the crest; the palpi are very narrow and short, and do not nearly touch
each other: this variation in the structure of the labrum and in the
size of the palpi, is very remarkable, considering how important, in a
classificatory point of view, these parts are in all other Cirripedes.
In the _mandibles_ there are either two or three main teeth, with the
whole lower part of the organ pectinated with sharp spines. _Cirri_:
the first pair is not short; in the individual examined, the two rami
had eleven and twelve segments. In the second pair, the shorter ramus
was two thirds of the length of the longer ramus, the segments being in
number ten and fifteen; in the arrangement of the spines this second
pair resembles its homologue in the three other species. In the third
pair, the two rami are very nearly equal in length, having sixteen and
eighteen segments; and the segments of the anterior ramus are only a
little thicker and more thickly clothed with spines than those of the
posterior ramus. The remaining cirri and the caudal appendages are as
in the other species.