Hab.—West Indies; Mus. Brit.—Jamaica, imbedded in a sponge; Mus. Cuming.
This is a remarkable species; when first seeing it imbedded in numbers in a sponge, I did not in the least doubt but that it was an [Acasta]: on examination, however, it is found to have a membranous basis, and therefore cannot by the definition enter into that sub-genus, to which, however, it is very closely allied. It differs from other sessile cirripedes very remarkably in the rostrum being nearly twice as long as the carinal compartments, so that the basis is always very oblique, or placed almost on one side; in this elongation of the rostrum, although in a different direction, we are reminded of [B. calceolus] and its allies; and these latter we know can hardly be separated from certain species of [Acasta]. Hence the position of our present species in this section, is not natural; but I am unable to place it elsewhere, without breaking down every definition: it should stand somewhat isolated, on one side of a line of affinity connecting [Balanus calceolus] and [navicula] with [Acasta purpurata].
General Appearance.—The shell is thin, fragile, smooth, and white, but covered to a considerable extent by a brown membrane, which on the sheath and opercular valve is of a bright tint, and clothed with bristles. Viewed laterally, the rostrum is seen to be considerably bowed, and from its being nearly twice as long as the other compartments, with its lower end bluntly pointed, the basal margin of the whole shell is rendered very oblique, forming a slightly concave line. The lateral compartments are rather longer, and about one third broader than the carino-lateral compartments. The rostrum, from terminating downwards in a blunt point, instead of being square or truncated, as in all other Cirripedes, and from the upper end being, as is usual, pointed, has, when disarticulated from the other compartments, the shape of a boat. The parietes are not at all porose: their internal surface sometimes shows traces of longitudinal ribs, but sometimes there are none. The radii are narrow, with their summits very oblique, and their sutural edges smooth. The sutural edges of the alæ are likewise smooth. The largest of Mr. Cuming’s specimens was .2 of an inch in diameter; but a disarticulated specimen in the British Museum must have been larger, having a rostrum .3 in length. The Basis is membranous.
The Scuta are rather convex; they have their lines of growth approximate, most finely crenated, so as to be very feebly striated longitudinally. Internally, the articular ridge is pretty well developed, its lower edge being very oblique; there is barely an adductor ridge: the pit for the lateral depressor muscle is deep. The spur of the Tergum is placed close to the basi-scutal angle of the valve; it is about half as wide as the valve, with the lower end truncated: sometimes it may be rather said to be bluntly pointed, owing to its carinal side sloping up to the basal margin. The articular ridge is pretty well developed. The crests for the depressor muscles are barely discernible.
Animal’s body unknown.
Section F.
Parietes and radii not permeated by pores; basis sometimes permeated by pores, sometimes not permeated, sometimes excessively thin and hardly distinguishable.