_Shell (when well preserved) dull purplish-brown: sutures formed by
oblique interfolding though rarely well developed: tergum with a deep
narrow pit, at the basi-carinal angle, for the depressor muscle._
_Hab._--Peru, Chile, Tierra del Fuego, Falkland Islands. Very common;
attached to littoral rocks and shells, and often associated with
_Balanus flosculus_, and sometimes with _Chthamalus cirratus_; Mus.
Brit., Cuming, W. Dunker, Darwin.
_General Appearance and Structure._--Shell generally depressed; when
growing crowded, sometimes cylindrical: colour dingy purplish-brown,
but when much corroded, dirty gray; very young shells are very dark
green, owing to the corium, which is of this colour, being seen
through the valves. Surface generally rugged, from irregular slight
longitudinal folds, and from the transverse overlapping tile-like
lines of growth; but sometimes the surface is nearly smooth, with
very slight longitudinal folds, these being gray coloured, the
intermediate parts being pale dingy purple, the shell thus becoming
striped. Orifice rhomboidal, passing into trigonal, owing to the
great width of the carinal end. Sutures generally very distinct,
rarely obliterated in the cylindrical varieties. Radii narrow,
generally exposing much of the alæ, which are plainly marked by
lines of growth: the radii themselves, when well developed, which
is not often the case, consist of small laminæ or ridges, placed
on both sides of the sutures, and interfolded or interlocked
together: usually only a trace of this structure is exhibited,
but occasionally, along some or along all the sutures (as in the
specimen figured), the laminæ of the radii interfold, as plainly as
in _C. intertextus_. The alæ differ slightly from the alæ of the
other species, in not forming so much of a rectangular projection,
the lower margin running with a gentle curve into the parietes. The
internal surface of the parietes is either smooth, or near the basal
margin is roughened with depending points: in some specimens from the
Falkland Islands, both edges of each suture were inflected, forming
a double ridge, with roughened edges, resting on the basal membrane,
and supporting the shell. I must mention that in my notes made at
these Islands, I remark that the basal membrane seemed sometimes to
be surrounded by a calcareous rim; none of the specimens brought home
are thus characterised; but bearing in mind the affinity of this
species to _C. intertextus_, no doubt we have here an indication
of the shelly ledge surrounding the basis, as described under that
species. The largest specimens which I have seen are a quarter of an
inch in basal diameter.
The _Opercular Valves_ generally have their summits much worn down.
The _scuta_ are elongated in the line of the longer axis of the
orifice; the articular ridge is very prominent, and is placed in
the middle of the tergal margin. The _terga_ are very narrow, as
in some varieties of _C. stellatus_: they are remarkable in two
respects, namely, in the depressor muscle being attached to a plate,
formed apparently by the union of the usual crests, parallel to the
outer lamina of the valve itself, a deep narrow cavity (fig. 2 _d_,
_p_) being thus formed; and secondly, in the far more extraordinary
circumstance of the existence of a small pit (_q_) at the extreme
basi-scutal corner of the valve, in which about half of the scutal
lateral depressor muscle is attached: I have observed no other
instance in any cirripede of the partial attachment of a muscle
properly belonging to one valve to another valve. The figures of
the valves 2 _b_, 2 _c_, 2 _d_, are from specimens most unusually
perfect, with the upper portion not worn away; the ordinary
appearance of the valves as seen from above, is given in fig. 2 _a_;
at fig. 2 _d_, a view is given of the tergum seen from vertically
beneath, showing the cavity for its own depressor muscle, and for
part of the lateral depressor muscle of the scutum.
_Mouth._--The crest of the labrum is hairy, without teeth: the palpi
have long spines at the end, but none along the inferior margin: the
mandibles have either four or five graduated teeth, the lower ones of
which are plainly double laterally: the maxillæ have a very sinuous
edge. _Cirri_: the first and second pairs are very short: on the four
posterior pairs the segments support either four or five pairs of
main spines, with the small intermediate spines rather larger than
usual.
_Branchiæ._--None; but where they ought to occur, there are two very
slight ridges clothed with hairs, about the 1/100th of an inch in
length. On the prosoma, there is a slight ridge, extending from the
base of the first cirrus towards the adductor scutorum muscle, also
clothed with hairs; this unusual character of the prosoma being hairy
is common to _C. dentatus_.
This species is the commonest cirripede on the shores of the Falkland
Islands: many of the specimens are there crowded together, and rendered
elongated and cylindrical, with the walls very thin, and the sutures
often obliterated; as the opercular membrane is very narrow, the
opercular valves are much influenced both in their outline and in
their crests and articulations, by the varying form of the shell: I
have even seen specimens with the scutum and tergum on one side twice
as large as on the other side.