_Rostrum very small, elongated, triangular: lateral compartments, each
with an aperture, and carina with two similar apertures, all four
leading into shelly tubular columns._
_Hab._--Attached to _Pollicipes mitella_, probably from the seas of
China; Mus. Brit.
_General Appearance._--This very singular shell would not, without
some examination, be thought to be a cirripede. From the symmetrical
position of the four apertures, with the diamond-shaped orifice in
the middle, with the sutures on each side of the rostrum, and from
its depressed and circular form, this shell bears some resemblance to
the perforated species of Scutella. Shell much depressed, generally
nearly circular, with the basal margin highly sinuous and even
sometimes almost branched. Surface slightly irregular, marked by
fine lines of growth, and covered by brown membrane. Of the four
compartments, the _Rostrum_ is very narrow, triangular, and comes up
to the orifice almost in a point: it is rather depressed, that is, it
lies rather below the level of the other compartments: the straight
sutures separating it from the lateral compartments are distinct in
the upper part, though always obliterated in old shells in the lower
part: these sutures are generally far plainer than those separating
the lateral compartment from the carina, which in most cases are
obliterated and calcified together, excepting close to the orifice.
The alæ of the rostrum are not externally visible, and there are no
radii to any of the compartments. The _Carina_ is twice as broad as
the rostrum, and is furnished with alæ of the usual shape, which are,
to a certain extent, externally visible. The _Lateral Compartments_
are broad, being broader than the carina; they are both penetrated,
down to the surface of attachment, by a hole or rather tube,--the
two holes standing opposite the rostral end of the operculum: the
carina is penetrated by two rather smaller but similar holes. It
is these four holes which give to this cirripede its very singular
aspect: they are rather smaller than the orifice of the shell; they
are oval, with their longer axes placed in the direction of the ray
of the circular shell: their manner of formation will be immediately
explained. The orifice is neatly diamond-shaped and broad: it is
rather small compared with the whole shell, and is closed by the
operculum, which is seated near the summit of the shell. Basal
diameter of largest specimen 2/10 of an inch; few, however, attained
this size, and perfect larvæ were included in much smaller specimens.
_Structure of Shell._--The rostrum (fig. 4 _c_) is remarkable from
its small size, and from the plainness of its sutures, in comparison
with those separating the other compartments, and this is exactly
the reverse of what I should have expected in a compartment tending
to become rudimentary. Not only is the rostrum small, but the alæ
project to an unusually small degree, and gradually slope away into
the lower part of the parietes. These peculiarities are even more
strongly marked in very young shells: thus in one of the size of a
pin's head, the rostrum consisted of a minute parallelogram, without,
as far as I could see, any alæ, and was only one fourth of the size
of the carina,--this latter compartment being only half as wide as
the lateral compartments. The carina, at this early period, had quite
distinct alæ.
The tubular prolongations from the four external holes are of course
very conspicuous on the under side of the shell (4 _b_); and their
structure is there plainly seen. In extremely young shells the holes
are not present; but very soon, at four points of the circumference,
namely, two in the carina and one in each lateral compartment, the
basal edge becomes indented, and during growth more and more deeply
indented; at last the horns or points of the bays, thus formed in
the circumference of the shell, grow inwards and meet, the four
indentations being thus converted into four rings or holes; as
the shell is added to, at its circumference, these come to stand
further and further from the exterior margin; and as the shell at
the same time rises above the surface of attachment, the holes are
added to at their basal edges, and are thus converted into shelly
tubes, generally freely open at the bottom as well as at the top.
Sometimes these tubes are closed at the bottom, and this is usually
caused by their sides having been added to in a spiral direction. A
somewhat sinuous double ridge or fissure, leading from the tubes or
holes to the exterior border, can always be perceived on the under
side of the shell. Occasionally, though rarely, in very old shells,
a second series of holes is formed outside the first four holes,
and often a tendency to this may be perceived in the just-mentioned
fissures expanding a little at their outer ends, thus forming four
new circumferential indentations. The purpose of this peculiar
structure, apparently, is to give support to the much depressed and
thin shell. In _Chthamalus stellatus_ and _scabrosus_, we have seen
a slight indication of a similar structure, in the formation on the
under side of the shell, but confined to the lines of sutures, of
obscurely tubular pillars: we have also something analogous in the
singular midribs, in Platylepas, causing the membranous basis to be
convex. I need only further add, that the parietes, in Chamæsipho,
are rather thin, and are composed of translucent shell, punctured for
the entrance of tubuli, with the punctures often arranged in straight
lines.
_Opercular Valves_ (fig. 4 _d_).--These are attached by a narrow
opercular membrane to the sheath, but little beneath the summit
of the shell. The _Scuta_ are considerably arched or convex: the
articular ridge is very prominent, and there is a thick strong
adductor ridge. The _Terga_ have a short, rather broad, rounded spur,
placed very nearly in the middle of the valve: the crests for the
tergal depressores are moderately developed, and are simple.
_Mouth._--The crest of the labrum is hairy, and is much hollowed
out. The palpi are small and narrow, with long bristles at their
apices. The mandibles have four or five teeth, with the lower part
pectinated. The maxillæ are notched. Of the _Cirri_, the second pair
is short, and all the segments are thickly clothed with bristles.
The third pair in the arrangement of the bristles resembles the four
posterior pairs. The segments in these pairs are elongated, and
support four pairs of main spines.