_Shell white, generally tinged in the upper part with pink; surface
irregular: parietal tubes very irregular: radii moderately wide, with
their summits oblique: alæ with very thick crenated sutural edges:
tergum with the spur not joined to the basi-scutal angle; spur with its
extremity equably rounded._
_Hab._--Philippine Archipelago; Barrier Reef, (Raine's Islet),
Australia; Mus. Cuming and Stutchbury. Attached to massive corals, to
coral-rock, to a Tridacna, and to _Tetraclita cœrulescens_.
_General Appearance._--Shell conical, moderately steep: white,
generally with a tinge of pinkish-purple in the upper part, owing to
the exposure of the tips of the upfilled parietal tubes. The lower
part of the surface is generally well preserved, and is formed by
very irregular, branching, longitudinal, slightly prominent, ridges.
Radii of moderate width, with their summits oblique. Orifice rather
large, rounded trigonal.
_Scuta_, rather narrow, with the upper part acuminated: the external
surface generally much disintegrated, and marked with some irregular
blotches of dark red. Articular ridge not prominent; articular furrow
rather deep: adductor ridge, distinct from the articular ridge,
pretty well developed, as are the crests for the rostral and lateral
depressor muscles. In young and well preserved scuta, there is an
external, medial, hyaline band, corresponding with the hollow under
the adductor ridge, and caused by the thinness of the valve along
this line.
_Terga_: externally, the carinal half is longitudinally and very
feebly striated. Internally, the articular furrow is very wide, but
shallow, and of unusual length, owing to the preservation of the
upper part of the valve; the articular ridge is not prominent. The
spur does not actually join, as in all the foregoing species, the
basi-scutal angle, but is separated from it by a short piece of basal
margin; its two sides are more nearly parallel than is usual, and
the end is regularly rounded. It is always rather narrow, though the
width varies considerably (fig. 3 _d_, 3 _e_). It extends in the same
straight line with the middle of the articular furrow. The terga,
though not possessing any striking characters, differ considerably in
appearance from those of the other species.
_Structure of the Shell and Radii._--The parietal tubes are
remarkable from their irregular shapes, and unequal sizes (fig. 3
_b_),--hardly two resembling each other. Sometimes a single elongated
tube will reach across the whole thickness of the walls. The septa
between the tubes are rather thick and rugged. The tubes are
generally darkly coloured from the adhering corium; they are solidly
upfilled, but only in the uppermost part, by dark chocolate-red
shelly matter. The radii are formed by irregularly branching ridges,
with the interspaces filled up solidly. The square edges of the alæ
are much thicker than in any other species, and are furnished with
transverse ridges, which are sometimes slightly branched. The inner
lamina of the walls near the basis, in most of the specimens, is
irregularly and longitudinally ribbed, the ribs being longitudinally
striated. The sheath and the upper part of the inner lamina of the
parietes are clouded with chocolate-red.
The animal's _Mouth_ and _Cirri_ were ill-preserved; but I was able
to make out that the labrum had some strong teeth, and that the
mandibles were furnished with five teeth, a greater number than in
any other species. The palpi had parallel sides as usual. In the
sixth cirrus, the segments had four pairs of main spines, instead of
the usual number of three.
_Affinities._--This species does not appear to be particularly
related to any other one: perhaps it is rather nearer to the two
following than to the foregoing species. The irregular parietal
tubes, thick-edged alæ, form of terga, five teeth to the mandibles,
and four pairs of spines to the segments of the posterior cirri, are
its chief characteristics.