_Shell faintly tinged with purple: radii broad, with their summits not
oblique: basis not porose. Scutum with the lines of growth crenated:
tergum with the spur extremely short, truncated, broad as half the
valve._
_Hab._--Raine's Islet, Barrier Reef, Australia, Mus. Stutchbury. Hab.
unknown, attached to and coated by Porites. Mus. Brit.
_General Appearance._--Shell conical, smooth, but with the lower part
sometimes narrowly ribbed in lines corresponding with the internal
longitudinal ribs; tinted pale peach-blossom purple, owing to the
sheath being finely so coloured; or wholly white. Radii broad, white,
square on the summit, hence orifice entire, ovate passing into
rhomboidal. The parietal portion of the carino-lateral compartments
extremely narrow, about one eighth of the width of the parietes of
the lateral compartments. Basis concave, partially imbedded in the
coral. Largest specimen .35 of an inch in diameter.
There are some specimens in Mr. Cuming's collection which appear to
belong to this species, and are certainly very closely allied to it,
but not having the opercula, cannot be identified positively; the
shell is flatter, with the walls strongly ribbed up to the orifice,
which is more rhomboidal: the basis is much more cup-formed and more
deeply imbedded in the coral; but these differences by themselves are
by no means sufficiently diagnostic.
_Scutum_: the lines of growth are crenated, causing the surface to
be very obscurely striated longitudinally: the articular ridge is
very prominent, as can be best seen from the outside, and runs down
the whole length of the tergal margin with a very regular curve, and
hence differs from the articular ridge in the foregoing species. The
adductor ridge is either absent, or very indistinct, and parallel
to the articular ridge: there is a deep little pit for the lateral
depressor muscle. _Tergum_ (7 _d_), with the apex somewhat produced
or beaked, and tinged purple: external surface almost flat, without
any longitudinal furrow: scutal margin curved. Spur very short,
placed quite close to the basi-scutal angle of the valve; broad as
half the valve; lower end square. Internally, the articular ridge is
prominent only in the uppermost part of the valve: crests for the
depressores very feeble.
_Parietes_: their internal surface is very strongly ribbed
longitudinally, the ribs being coarsely denticulated at their bases,
and finely fluted along their sides. The sheath is transversely
ribbed, and clothed with an epidermis furnished with transverse
rows of fine hairs. The _radii_ are of a dead white, whereas the
parietes are translucent; the summits are parallel to the basis;
they are broad; the radii of the carino-lateral compartments appear
extraordinarily broad, owing to the narrowness of the parietal
portion: the sutural edges are furnished with coarse septa, which
are sinuous, irregular, and obtusely denticulated; the interspaces
are filled up solidly. The _alæ_ are thin, with their sutural edges
almost smooth, and their summits oblique: in some specimens, during
the diametric growth, a mere, almost thread-like ribbon is added
to their sutural edges. _Basis_ slightly cup or saucer-shaped;
moderately thick, permeated by fine pores, and generally ribbed in
lines radiating from the centre. The walls and basis adhere together
very firmly.
_Mouth_: labrum with six teeth: mandibles with five teeth; the three
upper teeth being sharp, narrow, and unusually prominent; the two
lower teeth minute and sharp; maxillæ without a notch. _Cirri_ much
injured: first pair with one ramus apparently one third longer than
the other: segments not very protuberant: the posterior cirri have
elongated segments with five pairs of spines.
_Affinities._--This species is very distinct from all the foregoing:
in the carino-lateral compartments being so narrow, and tending, as
we may suppose, to become aborted; in the form and structure of the
whole shell, and in its habits, this species shows an affinity and
passage to the coral-inhabiting sub-genus Creusia, which has only
four compartments. There is also a close affinity to the sub-genus
Acasta. This species is so closely allied to the following, that
I at one time felt some doubts whether they ought to have been
specifically separated; it is also probably closely allied to _B.
terebratus_, but the materials hardly suffice for judgment: it is
also related, though less obviously, to _B. vestitus_.