_General Description._--Shell tubulo-conical; orifice of moderate size, rather deeply toothed; colour dull pink, or purplish, or dark flesh-colour; sometimes the radii are paler, sometimes of the same colour with the parietes. Surface smooth when well preserved, having transverse rows of minute spines. In the West Indian variety the walls are slightly or much folded, but I will describe this form separately. Size of largest specimen (Mus. Jeffreys), .6 of an inch in basal diameter. _Scutum_, with fine ridges radiating from the apex, and with the lines of growth, crenated: internally, the articular ridge is small, adductor ridge short and barely distinct: there is a rather deep and narrow pit for the lateral depressor muscle. The whole valve is much thinner than in _B. trigonus_, which in most respects it closely resembles. _Tergum_, with the apex pinkish purple, produced or beaked, but the beak is not needle-like, as in _B. psittacus_ and its allies, for the carinal margin is perfectly preserved up to the tip. Externally the valve is nearly flat, for the longitudinal furrow is very shallow. The spur is about one third of the width of the valve; its lower end is abruptly truncated: in European specimens (1 _b_) the whole basal margin, on the carinal side, slopes down to the spur in a straight line, which, together with the sharpness and production of the basi-scutal angle of the spur itself, gives to the whole valve a peculiar appearance: in the specimen (1 _c_) from the Lagulhas Bank, the basal margin on the carinal side is a little more hollowed out, but it is quite impossible to doubt about the specific identity of these specimens: in the West Indian variety (1 _d_) the basal margin on the carinal side forms a distinct but obtuse angle with the spur. In all cases the crests for the depressor muscles are very feebly developed. The _Compartments_ have their radii developed to a rather varying degree, with their summits oblique; hence the orifice is toothed: the sutural edges of the radii have their septa barely denticulated; the sutural edges of the alæ are smooth. The basis, as with the other species of this section, is permeated by pores; yet I found one specimen, from the Cape of Good Hope, with the basis apparently solid, thus offering a very singular anomaly. In the specimen imbedded in sponge, the basis, as viewed externally, is concave; whereas in Acasta, which always inhabits sponges, the basis is highly convex or hemispherical. The _Mouth_ and _Cirri_ resemble those of _B. trigonus_, and I can point out no distinguishing character.