_Shell internally imperial purple; parietes thick, with their internal
basal edges rough with irregular points and ridges; radii narrow; basis
very thin, solid. Scutum, with crests for the rostral and lateral
depressor muscles; tergum, with the end of spur rounded._
_Hab._--New South Wales, Sydney, Port Stephens, Moreton Bay; attached
to sandstone-rocks and shells, at low-water line; Mus. Brit., College
of Surgeons, Cuming, Stutchbury.
_Shell_ conical, very thick and very strong; longitudinally sulcated
more or less strongly; whole thickness of shell beautifully coloured
rich violet, or more strictly "imperial purple;" externally the
surface, from disintegration, is generally whitish; internally
the colour is best developed: the narrow radii and the thin basis
are white. The largest specimen which I have seen was one and
three-quarters of an inch in basal diameter, the walls close to the
basis being, in this instance, actually .3 of an inch in thickness.
_Operculum_ thick and strong, covered by yellowish-brown epidermis;
internally, the shelly substance is either all of the richest
purple or yellowish-white, tinged, especially in the upper part,
with purple. _Scuta_, with the apex beaked and somewhat reflexed;
articular ridge very thick, little prominent; articular furrow very
narrow, the impression made by the adductor muscle is seated very
high up the valve; there is hardly an adductor ridge, but the surface
of the valve is angularly prominent in a curved line, running from
the articular ridge to near the rostral angle of the valve. At the
rostral angle, the occludent margin is not folded inwards, as is
generally the case, but the surface is flat, and is marked by four
or five crests for the attachment of the rostral depressor muscle.
There are other crests for the lateral depressor muscle. _Tergum_,
with the apex somewhat produced and beaked, but blunt; longitudinal
furrow shallow; spur of moderate breadth, with its lower end rounded;
the basal margin on the carinal side of the spur slopes towards it.
Internally, articular ridge moderately prominent. Crests for the
tergal depressor well developed.
_Parietes_, solid, thick, with the basal internal edge (4 _c_) formed
of short ridges, or flattened and irregular points, which in very old
specimens are but little prominent; inner surface, finely, closely,
and irregularly ribbed longitudinally, but in some specimens nearly
smooth. The _radii_ are nearly white; they are narrow, sometimes
hardly at all developed, and have their summits very oblique and
jagged; exteriorly, they are sulcated in a transverse direction, and
sometimes form oblique steps, from having been formed layer over
layer: their sutural edges are formed of irregularly branching crests
or septa. The _alæ_ have their summits very oblique; their sutural
edges are thick and crenated: the part added during diametric growth
on the inner surface is smooth, and has a different appearance from
the transversely ribbed portions of the sheath, of which the alæ
form a portion. The lower edge of the sheath is hollow beneath. The
carino-lateral compartments are very narrow.
_Basis_ calcareous, thin, white, sometimes opalescent, apparently
formed by an aggregation of very minute calcareous beads, with no
trace of furrows radiating from the centre.
_Mouth_: labrum hairy, with apparently some very minute teeth;
mandibles, with the fourth and fifth teeth small and rudimentary;
maxillæ rather broad, with a narrow and rather deep notch under the
two great upper spines: outer maxillæ with the lower lobe very large.
_Cirri_: first pair, with the rami unequal by several segments:
second pair, with the rami unequal in length by about six segments:
third pair elongated, with the segments very numerous, almost
equalling those in the sixth cirrus; upper segments of both rami much
elongated, each with only a circle of spines; segments in the above
first three pairs of cirri only slightly protuberant. Posterior cirri
elongated, with the upper segments bearing three pairs, and the lower
segments four pairs of main spines, between which there is a small
intermediate tuft.
_Affinities._--This noble Balanus, in all the characters derived
from its opercular valves, and from its cirri, is closely allied to
the last species: in the structure, however, of the shell and of
the basis, it comes closer to the following, _B. flosculus_. The
crests on the under side of the scutum, for the lateral depressores,
are confined to these three species; and the crests for the rostral
depressores are confined to _B. imperator_ and _vestitus_, but they
are generally rudimentary in the latter. The internal basal structure
of the parietes is singularly like that of _Chelonobia caretta_,
though there is no other special affinity to that genus. In the
nature of basis; in the structure, to a certain limited extent, of
the walls of the shell; in the narrowness of the carino-lateral
compartments; in the elongation of the third pair of cirri; in the
crests for the rostral and lateral scutal depressores, _B. imperator_
comes nearer to the genus _Tetraclita_ than does any other species of
Balanus.