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.