These plates are of a faint-brown or yellowish colour, and are conspicuous: the degree of calcification differs considerably; some are quite brittle and very thin, others half horny, and effervesce only slightly in acids. After having been placed in acid, there is no apparent difference between the parts before occupied by the calcified plates and the surrounding membrane; these plates, however, are not superficial, but consist of several of the laminæ, which together compose the ordinary integument, in a calcified condition. Like the integuments of the body, and unlike the valves of the capitulum, these calcified plates are thrown off at each exuviation. Neither the exact shape nor number of the plates corresponded in different individuals, nor even on opposite sides of the same individual. The margins of the plates often have a sinuous corroded appearance; they are, moreover, often penetrated by minute rounded holes, that is, by minute, rounded, non-calcified portions. In one specimen from the Antarctic expedition, there were only here and there a single shield on the segments of the posterior rami, and no plate on the prosoma. Of two specimens in another and older set in the British Museum, from an unknown locality, both had shields on the segments of the cirri, but only one had the large plate on the prosoma. I may here mention that in one specimen, in which the calcified plates were most developed, and which was nearly ready to moult, there were, within the filamentary appendages on the prosoma, small irregular balls of calcareous matter, appearing to me as if calcareous matter had been morbidly excreted, and not like a provision for the future. _Range._--This species, in the present state of our knowledge, seems to range further than any other of the genus, extending from Upper California, (lat. 32° to 35° N.,) across the Pacific, to at least 32° S., perhaps much farther south, for it was collected during the Antarctic expedition, and 32° was the highest latitude traversed by that expedition. _Affinities._--This species is closely related to _P. cornucopia_ and _P. elegans_, but differs rather more from them, than these two do from each other. In the capitulum the chief distinctive characters are--the more perfect graduation in size, and the greater number, (taking equal-sized specimens,) of the whorls of latera--the darker colours--the central part of the basal margin of the carina in this species, being considerably excised--the peculiar form of the basal margin of the scuta--and lastly, the scutal margin of the terga being more hollowed out. In the animal’s body, the most obvious distinctive character is the uniarticulate caudal appendage. This species agrees with _P. elegans_, in the presence of the singular elbowed teeth, on some of the spines in the first three pairs of cirri. 4. POLLICIPES MITELLA. Pl. VII, fig. 3. LEPAS MITELLA. _Linn._ Systema Naturæ, 1767.