cirri, as on the three posterior pair. There are some exceptions to the foregoing general rules: in the posterior cirri of _Alepas cornuta_, there is only one pair of long spines to each segment (fig. 28); in _Dichelaspis Lowei_, there are eight pair; in _Lepas fascicularis_, in old specimens, the segments are paved with a triangular brush of spines; the upper segments in _Pæcilasma eburnea_ support small oblong brushes; and, lastly, in _Pæcilasma fissa_ (fig. 29), and _crassa_, the spines form a single circle round each segment, interrupted on the two sides. These spines are often doubly serrated or plumose: many of them on the protuberant segments of the first three pair of cirri, are sometimes coarsely and doubly pectinated. _Caudal Appendages._--These are present (Pl. X, figs. 18 to 24) seated on each side of the anus, in all the genera, except in Conchoderma, Anelasma, and _Scalpellum villosum_; they consist of a very small single segment, destitute of spines in Lepas, and spinose in Pæcilasma, Dichelaspis, Oxynaspis, Scalpellum, and some species of Pollicipes; they consist of several segments in Alepas, Ibla, Lithotrya, and in some species of Pollicipes. In the latter genus, some species have their caudal appendages multiarticulate, though so obscurely articulated, that the passage (fig. 22) from several to one segment is seen to be easily effected. When the appendage consists of many articulations, it is generally about as long as the pedicel of the sixth cirrus; but in _Ibla quadrivalvis_, it is four times as long. The segments are narrow, slightly flattened, much tapering; each (fig. 24) is surmounted by a ring of short spines, which are generally longest on the apex of the terminal segment. I could never trace muscles into these appendages.