BALANUS NIGRESCENS. _Lamarck_, (1818) in _Chenu_. Illust. Conch.,
Tab. 4, fig. 16.
---- GIGAS. _Ranzani._ Memoire di Storia Nat., 1820, Tab. 3, fig. 5,
6, 7.
---- ---- _De Blainville._ Dict. des Sc. Nat., Tab. 116, fig. 2, 2
_a_.
_Shell cinereous, tinted with pale or blackish blue, or wholly white.
Scutum with the articular ridge terminating downwards in a small,
sharp, free point: adductor ridge prominent. Tergum with the apex
produced and needle-like._
_Hab._--Swan River, West Australia, Mus. Brit., attached to
sandstone. Attached to sandstone and to each other in a group, Mus.
Cuming. Twofold Bay, S. E. Australia, attached to tidal rocks and
Patellæ, Mus. Darwin.
There can be no doubt that this species is the _B. nigrescens_ in
Chenu, who had access to Lamarck's original specimens; and there can be
equally little doubt that it is the _B. gigas_ of Ranzani, collected,
during Baudin's expedition, at King George's Sound: it is essentially
allied to _B. psittacus_, but in external appearance strikingly
resembles some of the varieties of _B. tintinnabulum_.
_General Appearance._--Shape tubulo-conical: walls smooth, sometimes
longitudinally ribbed: colour ashy-gray tinged with blue, but many
specimens are dark purplish-blue, owing to the disintegration of the
outer lamina, and consequent exposure of the almost solidly filled
up, dark blueish parietal tubes; on the other hand, some specimens
are quite white. Ranzani describes the colour as earthy-violet, which
is very characteristic of some of the specimens. The orifice is apt
to be rather small, compared to the size of the specimens, and tends
to be hexagonal. The radii are often rather narrow. The opercular
valves are tinted pale blue. The basal diameter of the largest
specimen is two inches, and its height two and a quarter.
The _Scuta_ have their basi-tergal corner much rounded off, as in
_B. psittacus_, so that the tergal margin does not extend more than
half down the valve. The surface is somewhat prominent, along a line
running from the apex to the point of chief curvature in the basal
margin. The surface is not striated. Internally, the articular ridge
is little prominent, and not reflexed; the lower end depends as a
free, sharp style or point. The adductor ridge is moderately sharp,
and stands some little way distant from the articular ridge: it is
produced downwards, and forms a moderately deep and large cavity for
the depressor muscle; but this cavity is not closed, and does not
extend up, as in the two last species, to the apex of the valve.
_Terga_, narrow, with a sharp, prominent, needle-like beak. Spur,
long, narrow, placed at less than its own width from the basi-scutal
angle: the basal margin on both sides slopes down to the spur: the
scutal margin is not inflected. Internally, the articular ridge is
very feebly developed, but extends down close to the basi-scutal
angle. On the under surface in the upper part of the valve, there is
a short, very slight ridge, extending on the carinal side, near and
parallel to the articular ridge; this slight ridge plays an important
part, as in the two foregoing species, in the formation of the beak
or apex. Crests for the depressor muscle are hardly distinguishable.
The _Walls_ appear to vary in some degree in strength and thickness;
as is likewise the case with the opercular valves. In some of the
thinner specimens, the parietal tubes are large, and the longitudinal
septa are furnished with small, sharp denticula. The tubes are often
thickly lined or almost filled up solidly with blue shell; they are
not crossed by transverse septa.
The _Radii_ vary in width; externally they are often finely ribbed
transversely, at other times they are smooth; their septa are fine
and thin, with their delicate denticuli not extending to the outer
lamina: they are very porose. The _alæ_ have their summits parallel
to the basis; their sutural edges are most finely crenated. The
sheath is blueish, excepting the wedge-like portions of the alæ which
have been added during the diametric growth, and these are white.
_Mouth_: labrum without teeth: mandibles with five sharp teeth:
maxillæ with the edge straight. _Cirri_, first pair with the rami
very slightly unequal; segments of the shorter ramus and of both rami
of the second pair protuberant: posterior cirri with the segments
shield-shaped in front, bearing four pairs of spines, of which the
upper pair is much longer than the lower pairs; each pair has a small
intermediate tuft of minute spines.