_Basis membranous: caudal appendages none._
_Hab._--New South Wales (Twofold Bay), Mus. Darwin; Swan River (?),
Mus. Cuming. Attached to littoral rocks and shells, and associated
with _Tetraclita purpurascens_, _Balanus nigrescens_, _Chthamalus
antennatus_, _Chamæsipho columna_.
_General Appearance._--Shell nearly circular, moderately or slightly
depressed; colour gray. The eight normal compartments of the inner
whorl are two or three times as large as those in the second whorl;
the other smaller compartments or scales graduate very regularly in
size, to mere beads, at the extreme basal edge, and are arranged
symmetrically. The general aspect of the shell depends chiefly on
the degree to which the surface has been disintegrated, and differs
greatly in the two extreme states. When well preserved, the general
appearance is very elegant; the scales are all neatly imbricated;
they terminate upwards in points, but with their extreme tips
generally broken; from laterally overlapping each other, their
external surfaces (as seen, when corroded, in fig. 4 _d_, or when
slightly exposed after the continued growth of their basal margins,
in fig. 4 _a_) become longitudinally keeled, often with a secondary
ridge or shoulder on one side; they are, also, crossed by rather
conspicuous and regular lines of growth, or more strictly, former
lines of union between the several valves: the orifice of the shell
in this perfect condition is sub-rhomboidal and notched, and the
scuta are united to the terga by nearly straight sutures. On the
other hand, when the shell has been considerably corroded, and this
seems to be the more common condition, the appearance is not elegant:
the scales in the successive whorls are not imbricated, but owing to
their upper parts having been worn down, they present a tesselated
surface (4 _d_), with the tesseræ graduated in size, and of a
peculiar shape, namely, a rectangle, with a more or less broad square
projection on the exterior side, together often with a large square
notch on one or both corners, caused by their laterally overlapping
each other. Some of the shells are so deeply corroded, that no
portion of the original surface is preserved, excepting the lowermost
bead-like scales; and the whole shell has so rugged an aspect,
that the successive whorls of the worn-down valves might easily be
overlooked. In the corroded specimens, the orifice approaches to
circular in outline, and is large and nearly entire: the scuta and
terga are deeply interlocked together. The largest specimen which I
have seen was one inch and a quarter in basal diameter.
_Structure of the Shell._--The eight inner normal compartments are
not thicker than the outer valves, and are far thinner than in
ordinary sessile cirripedes. They are arranged as in Octomeris.
The lateral compartments are broader than the rostro-lateral and
carino-lateral compartments. The shelly laminæ, of which these
compartments and the opercular valves are composed, alternate with
yellow membranous layers, exactly as is the case with Octomeris.
The lateral edges of the compartments, beneath the alæ, in the six
compartments having alæ, bend inwards, especially just above the
basis, so that the compartments in their lower parts (fig. 4 _c_)
stand much more separate than is usual. The basal edge of each is
irregularly toothed. The sheath presents no particular character.
The eight compartments have longitudinal shoulders and ribs on their
external surfaces, caused by the lateral overlapping and pressure of
the exterior scales. They are also marked by slight, transverse or
oblique calcareous ridges, caused by the attachment of the membrane,
by which they are united to the smaller compartments or scales
outside them. A new line of attachment, and consequently a new ridge
is formed, lower and lower down at each period of growth, as the
shell is added to at the basis,--in the same manner as new ridges are
added to the lower edge of the sheath at each period of growth. The
shell, excepting in old worn-down specimens, increases largely by
diametric growth: during the diametric growth, the outer scales must
be laterally separated a little from each other, and probably they
are laterally added to; but there are no distinct lines of suture,
or rows of smaller scales, corresponding with the sutures between
the eight inner compartments. The alæ do not project much; their
edges, as well as the shoulders into which they fit, are generally
irregularly crenated: they are added to during diametric growth
above the line of attachment of the opercular membrane. There is no
appearance of radii; but as the eight inner compartments are added to
laterally, and are often crenated, on the edges which correspond with
the radii in other Cirripedes, such edges must be considered as radii.
With respect to the scales in the several outer whorls, they resemble
each other except in size, and the outermost scales are reduced to
mere transversely elongated beads. Their basal edges are concave
inwards, being bent like the ridge of a house; hence sub-triangular
spaces or tubes, lined by the corium, run up between the scales. When
perfectly preserved, the outline of each scale is a much elongated
triangle, but usually, from their summits having been worn off, the
outline is nearly that of a parallelogram. Their basal edges are
dentated, and their upper parts, both on the outside and inside, are
marked, where joined to the other compartments, by slight calcareous
ridges: outside, there are longitudinal shoulders (Pl. 20, fig. 4
_b_), caused by the lateral overlapping of the adjoining scales;
these are best seen in section in corroded specimens (fig. 4 _d_):
on the inside there are, also, in the upper part, slight medial
longitudinal ridges, caused by the sutures, which the scales have
covered.
In large old specimens there are ten, or even more, whorls of
compartments, but it is scarcely possible to count them with
any accuracy. The first whorl consists of the eight large inner
compartments, though, homologically, it is doubtful whether the
rostro-lateral compartments in any sessile cirripede really belong
to the same whorl with the others. The second whorl consists of
eight smaller pieces, covering the eight sutures in the first whorl
(see the tracing of the basal edges of all the compartments and
valves in a very perfect specimen, Pl. 20, fig. 4 _c_). The third
whorl, in large and perfect specimens, consists of twice the number,
or sixteen, still smaller scales, corresponding with the sixteen
sutures of the second whorl; but sometimes there are less than
sixteen pieces, owing to some of the scales being large enough to
cover two adjoining sutures as well as the intermediate portion of
the compartments of the first whorl. In the fourth whorl, instead
of there being, even in the most perfect specimens, twice sixteen,
or thirty-two pieces, there are only twenty-four; this being caused
by single pieces (placed alternately with two pieces) being broad
enough to cover two sutures as well as the intermediate portion of
the compartment of the third whorl. In the succeeding whorls this
same arrangement seems to be the usual one, so that in the fifth
whorl, instead of there being, in the most perfect specimens, twice
twenty-four, or forty-eight pieces, that is, twice the number in the
last whorl, there are only thirty-six scales, or once and a half as
many scales.
The _Basis_ is thin and membranous; it firmly adheres to the surface
of attachment.
_Scuta_, nearly flat: the articular ridge is very prominent, and
there is a deep articular furrow both above and below; but the
precise outline of the ridge and furrows varies: there is a pit
for the adductor muscle, but no crests or marks for the other
muscles. The _Terga_ are remarkable from the extreme prominence of
the articular ridge and depth of the articular furrow. The basal
margin viewed internally seems straight, so that there appears to be
no spur; but viewed externally, when the crests for the depressor
muscles are seen to depend considerably beneath the true basal
margin, a slight, very broad spur may be perceived to exist. These
depending crests for the muscles are rather thin, but they extend
over half the basal margin of the valve.
_Mouth._--The labrum is very bullate, being as long in its
longitudinal axis as the rest of the mouth: crest hairy, with some
very minute teeth. Palpi truncated, with their apices not nearly
touching each other; thickly clothed with spines. Mandibles with
three large single teeth, of which the lower one has a single
fine tooth at its upper basal edge, showing a tendency to become
pectinated; inferior part short, coarsely pectinated. Maxillæ
notched, with a slight second notch and slight double prominence in
the lower part.
_Cirri._--First and second pairs short, with the rami in each
unequal in length by about four segments; on both rami in the second
cirrus, and in the shorter ramus of the first cirrus, there are some
coarsely pectinated spines. All four posterior cirri are alike; the
segments bear five pairs of strong spines, with a large intermediate
tuft of fine spines: the dorsal tuft is also large, consisting of
short thick, and long finer spines. There is no vestige of _caudal
appendages_, though present in the succeeding species.
Branchiæ moderately large, in area equalling the prosoma; surface not
plicated.