Valvæ ab 18 usque ad 100 et amplius: lateribus verticilli inferioris multis; lineis incrementi deorsùm ordinatis: sub-rostrum semper adest: pedunculus squamiferus.

Valves from 18 to above 100 in number: latera of the lower whorl numerous, with their lines of growth directed downwards: sub-rostrum always present: peduncle squamiferous.

Hermaphrodite; filamentary appendages either none, or numerous and seated on the prosoma and at the bases of the first pair of cirri; labrum bullate; trophi various; olfactory orifices generally highly prominent; caudal appendages uni-articulate and spinose, or multi-articulate.

Attached to fixed, or less commonly to floating objects, in the warmer temperate, and tropical seas.

It has been remarked, under Scalpellum, how imperfectly that genus is separated from Pollicipes; and we have seen under [Scalpellum villosum] that the addition of a few small valves to the lower whorl, would convert it into a Pollicipes, most closely allied to P. sertus and spinosus. It has also been shown, that the six recent species of Pollicipes might be divided into three genera, of which P. cornucopia, P. elegans, and P. polymerus, would form one thoroughly natural genus, as natural as Lepas and the earlier genera; P. mitella would form a second; and P. sertus and P. spinosus a third; but I have acted to the best of my judgment in at present retaining the six species together. As far as the valves of the capitulum are concerned, it would be very difficult to separate P. mitella from P. sertus and spinosus.

Description. The number of valves in the capitulum has in this genus acquired its maximum. The number varies considerably in the same species, and even on opposite sides of the same individual, and generally increases with age. It is more important, that the number of the whorls in P. cornucopia, and in the two following closely-allied forms, also increases with age. In P. sertus and P. spinosus, even the number of the whorls varies in different individuals, independently of age. The valves are arranged alternately with those above and below; they are generally thick and strong, making the capitulum somewhat massive; in some species they are subject to much disintegration; but in others, the apices of the several valves, especially of the carina and rostrum, are well preserved, and project freely: they are covered with membrane, which, differently from in most species of Scalpellum, either does not bear any spines, or only exceedingly minute points. In all the species there is a sub-rostrum and sub-carina, and often beneath these a second sub-rostrum and sub-carina. In medium-sized specimens there are at least 20 valves in the lowermost whorl. The carina is either straight or curved, but never rectangularly bent, and is always of considerable breadth. None of the valves are added to at their upper ends. The scuta have a deep pit for the adductor muscle. The valves lie either some little way apart, or more commonly close together. In P. mitella the scuta and terga are locked together by a fold, and the valves of the lower whorl overlap each other in a peculiar manner, resembling that in which the compartments in the shells of Sessile Cirripedes fold over each other.

The Peduncle is of considerable length in some of the species, and rather short in others; it is, in every case, clothed with calcified scales. The scales in the first four species are placed alternately and symmetrically; they are formed and added to in the same manner as in Scalpellum; they differ in size according to the size of the individual, and consequently the lower scales on the peduncle, formed when the specimen was young, are smaller than the upper scales; the lower scales are separated from each other by wide interspaces of membrane, owing to the continued growth of the peduncle by the formation of new layers of membrane, and the disintegration of the old outer layers. Each scale is invested by tough membrane (or has been, for it is often abraded off), in the same manner as the valves; each is furnished with one or more tubuli, in connection with the underlying corium. In P. sertus and P. spinosus, the scales are small, spindle-shaped, and not of equal sizes, and the rows are distant from each other, so that their alternate arrangement is not distinguishable; in these two species, new scales are formed round the summit of the peduncle, and the growth of each is completed whilst remaining in the uppermost row; but, besides these normal scales, such as exist in the other species of Pollicipes and in Scalpellum, new scales are formed in the lower part of the peduncle, which are generally of very irregular shapes, are often larger than the upper ones, are crowded together, and sometimes do not reach the outer surface of the membrane. This formation of scales in the lower part of the peduncle, independently of the regular rows round the uppermost part, is perhaps a feeble representation of the calcareous cup at the bottom of the peduncle in the genus Lithotrya. The prehensile antennæ will be described under P. cornucopia.

Size.—Most of the species are large: and P. mitella is the most massive of the Pedunculated Cirripedes.

The Mouth is not placed far from the adductor muscle. The labrum is highly bullate. The mandibles have either three or four main teeth ([Pl. X], [fig. 1]), with often either one or two smaller teeth inserted between the first and second. The maxillæ ([Pl. X], [figs. 13, 14]), have their edges either straight and square, or notched, or more commonly with two or three prominences bearing tufts of finer spines. The outer maxillæ ([fig. 17]) generally have a deep notch on their inner edges, but this is not invariable. The olfactory orifices in most of the species are highly prominent.

Cirri.—The first pair is never placed far distant from the second. The posterior cirri have strong, somewhat protuberant segments; and between each of the four or five pair of main spines ([Pl. X], [fig. 27]), there is a rather large tuft of straight, fine, short bristles. The second and third pair have the basal segments, either of the anterior rami, or of both rami, so thickly clothed with spines ([fig. 25]), as to be brush-like: in P. mitella, however, the third pair is like the three posterior pair in the arrangement of its spines, in this respect resembling the sessile Chthamalinæ. The caudal appendages are either uni-articulate and spinose, or multi-articulate: it is remarkable that there should be this difference in such closely allied species as P. cornucopia and P. polymerus: the short, obtuse, obscurely-articulated caudal appendage of the former species ([fig. 22]) makes an excellent passage from the uni-articulate ([fig. 19]) to the multi-articulate form, as in P. mitella.