_Shell tinged with orange; viewed externally seems formed of only four
compartments, owing to the carino-lateral and lateral compartments on
each side being separated only by an obscure fissure._
_Hab._--New South Wales; apparently from deep water, attached to
sandstone.[135]
[135] I am indebted to Mr. Bowerbank for this unique and
interesting species, which I have deposited in the British Museum.
_General Appearance._--Shell conical, smooth, with a tinge of orange
colour; orifice large, deeply notched, sub-triangular. Viewed
externally, the compartments in appearance are only four; but on
close examination, the lateral compartments are seen to be divided
by a very fine fissure into two nearly equal compartments. The
rostrum is broad and flat, and when carefully examined, it also is
seen to be divided by two fine fissures into three compartments;
of these the middle one, or true rostrum, is a very little broader
than the rostro-lateral compartment on each side; hence, on careful
examination, the shell is found to consist of eight compartments. The
carina is much compressed. The radii (in my one specimen) are not
developed. The carina, and the two lateral compartments alone, have
alæ; for the carino-lateral compartments are too closely joined to
the lateral compartments, and the true rostrum is too closely joined
to the rostro-lateral compartments, to have their alæ developed. The
four alæ which are developed, are very broad, widely exposed, and
marked externally by lines of growth. The basal diameter of my one,
apparently old specimen, is one inch.
_Structure of Shell, &c._--The inner surface (fig. 1 _b_) of the
rather thick parietes is smooth, except close to the base, where
it is roughened by a few irregular points. The sheath descends
low down, and has its lower edge slightly free. The sutures are
much plainer on the internal than on the external surface, and
can be here plainly seen to be eight in number. That portion of
the carino-lateral compartment, which helps to form the sheath,
is narrow, though the whole compartment is of very nearly equal
width with the lateral compartment. It is a singular fact, that no
portion of the rostro-lateral compartment helps to form the sheath;
for the alæ of the lateral compartments, overlap the whole upper
part of the rostro-lateral compartments, and abut against the true
rostrum. Hence, when the sheath alone is examined, the number of
the compartments appears only six. In a section the true rostrum
can be seen to underlie the rostro-lateral compartments, and thus
exhibits vestiges of alæ. The fact of the rudimentary rostro-lateral
compartments not forming a part of the sheath offers a marked
difference from Chelonobia, which otherwise has this part of the
shell very similarly constructed. The diametric growth of the shell,
which seems to be considerable, is effected by the four large alæ of
the carina and of the lateral compartment on each side. The sutural
edges of the alæ are added to in a regularly inclined line down to
the basis. The _basis_ is calcareous, and not very thin.
Both opercular valves (fig. 1 _c_, 1 _d_) closely resemble those of
the last species.
The _Scuta_ are remarkably narrow and elongated: the external
surface is slightly furrowed longitudinally, the prominent lines
of growth are much wrinkled. The articular ridge is blunt, and not
very prominent. The upper part of the valve is reflexed, and a
considerable portion must have projected freely. The _Terga_ are
nearly twice as broad as the scuta: the carinal half of the valve is
very smooth, and is coloured beautiful reddish-orange. The spur is
tolerably distinct. The articular ridge is not very prominent.
The _Mouth_ resembles that of the last species. The mandibles have
three sharp teeth, with the inferior part narrow and pectinated
with long spines: the edge, also, between the upper main teeth, is
pectinated with short spines, which latter often have their summits
crenated. The _maxillæ_ are notched; the inferior corner is produced
into a small step-formed projection. The outer maxillæ are more
pointed than is usual.
The _Cirri_ most closely resemble in every detail those of _P.
giganteum_; I can point out only one slight difference, namely, that
the lower segments, in the posterior ramus of the third cirrus, which
are thickly clothed with spines, are more numerous in proportion to
the upper segments with the spines arranged in regular pairs (being
as 4 to 14), than in _P. giganteum_, in which only one fifth (or 4 to
20) are so clothed.
_Caudal Appendages._--These are very small, not being more than
one fifth of the length of the pedicel of the sixth cirrus: their
segments are indistinct, and they support a very few coarse spines.
The rami of the sixth cirrus, in the one specimen, had twenty-five
segments, whilst each caudal appendage had only five. Hence the
caudal appendages are far less developed than in _P. giganteum_.