_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_. The stomach, in those species which I opened, is destitute of cæca; the hepatic glands are arranged in straight lines; the rectum is unusually short. The prosoma is well developed. In _P. cornucopia_, _P. elegans_, and _P. polymerus_, there are numerous filamentary appendages both on the prosoma, and at the bases of the first pair of cirri: these appendages are occupied by testes, and I suspect stand in relation to the length of the peduncle and consequent great development of the ovaria. In order to give space for the filamentary appendages, the sack (generally roughened by small inwardly-pointing papillæ) penetrates more deeply than usual into the upper part of the peduncle. There are small ovigerous fræna in _P. sertus_, _P. spinosus_, and _P. mitella_: in the three other species, the frænum or fold occupies the usual position on each side, and is large; but in one specimen carefully examined by me, I was unable to see any glands; and in another specimen, the ovigerous lamellæ were not attached to the fræna; hence I conclude that the fræna are functionless in these three species. _Affinities._--I have already remarked on the close relationship between this genus and Scalpellum; there is also some affinity with Lithotrya. _Distribution._--All over the world. The _P. cornucopia_ ranges from Scotland to Teneriffe: the _P. polymerus_ is found in opposite hemispheres in the Pacific Ocean, extending from California to at least as far as 32° south of the Equator. _Geological History._--Having so lately given, in the ‘Memoirs of the Palæontographical Society,’ a full account of all the fossil species known, I will not repeat here the conclusions there arrived at. I will only state, that species of Pollicipes are found in all the formations, extending from the Lower Oolite to the Upper Tertiary beds. 1. POLLICIPES CORNUCOPIA. Pl. VII, fig. 1. POLLICIPES CORNUCOPIA. _Leach._ Encyclop. Brit. Supp., vol. iii, 1824.