markable 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 caeca; 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 papillae) penetrates more deeply than usual into the
upper part of the peduncle. There are small ovigerous fraena in _P.
sertus_, _P. spinosus_, and _P. mitella_: in the three other species,
the fraenum 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 lamellae were not
attached to the fraena; hence I conclude that the fraena 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 deg. south of the Equator.

_Geological History._--Having so lately given, in the 'Memoirs of the
Palaeontographical 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.

---- SMYTHII, var. _Leach_. Ibid.

LEPAS POLLICIPES. _Gmelin._ Systema Naturae, 1789.

---- GALLORUM. _Spengler._ Skrivter Naturhist. Selska