_Shell steeply conical, having four longitudinal gray bands placed
crosswise: radii with their summits oblique: basis thin, solid. Scutum,
with the lines of growth smooth; no distinct pit for the lateral
depressor muscle: tergum as in B. cepa._
_Hab._--East Indian Archipelago, attached to lamelliferous corals,
and associated with _Pyrgoma grande_ and _Creusia spinulosa_, Mus.
Brit. and Stutchbury and Darwin. Philippine Archipelago, attached to
a Tetraclita, Mus. Cuming.
I have seen four sets of specimens of this species, taken in four
different places, one set containing above twenty individuals, and
all resembled each other exactly: nevertheless, this species comes so
close to _B. cepa_, that I am somewhat doubtful about its specific
distinctness.
_General Appearance._--Shell smooth, or slightly folded, steeply
conical; white, with four longitudinal bands of pale brownish-gray
colour, namely, on the rostrum, the carina, and the two lateral
compartments: the carino-lateral compartments are very narrow and
almost white: the four brownish-gray bands are darkest in the upper
part of the shell, though always rather faint, and die out towards
the base: they can sometimes be seen to be formed of several narrow
longitudinal stripes; the tint shows a trace of containing purple.
The orifice of the shell is small, rhomboidal, and not quite entire,
owing to the obliquity of the summits of the moderately broad radii.
In structure, the shell, radii, and alæ resemble those in the last
species. The basis, however, does not appear to be permeated by
pores. Basal diameter of largest specimen .25 of an inch.
The _Scuta_ most closely resemble those of _B. cepa_, but the lines
of growth are not crenated, and internally there is only a very
minute pit for the lateral depressor muscle, placed almost on the
edge itself of the valve. The _Tergum_ hardly differs at all from
that of _B. cepa_, but is perhaps of rather greater breadth.
The _Mouth_ does not differ from that in the last two species. In the
_Cirri_, the three posterior pairs have elongated segments, bearing
only three pairs of spines, of which the lowest pair is minute: in
_B. allium_, and I believe in _B. cepa_, there are five pairs of
spines on each segment.
This species differs from the last only in the peculiar colouring,
smoother walls, more oblique radii, solid basis, and more especially
in the scuta having the lines of growth not crenated, and internally,
in the pit for the lateral depressor muscle being so very minute and
placed on the basi-tergal edge of the valve. The posterior cirri,
also, I believe, differ in the number of the spines which the segments
support; nevertheless, I cannot feel confident about the specific
distinctness of _B. quadrivittatus_.