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.