Similar in all respects to _C. maxwelli_, except that it is rather smaller, and that the brown colour of the back darkens to black on and at each side of the base of the tail, below which there is an abrupt change to white on the backs of the hams. Female with horns. Horns short, but almost as long in the female as in the male, placed in the same straight line as the nasal profile, slightly incurved: those of male about 1½ inch long, their basal diameter going about 2½ times in the length; those of female about 1¼ inch long, basal diameter going about 3 times in the length. Dimensions:--Height much as in _C. monticola_, length of ear 1·6 inch, hind foot 6·1. Skull (♂): basal length (c.) 4·6 inches, greatest breadth 2·4, anterior edge of orbit to muzzle 2·5. _Hab._ Africa south and east of the Niger, extending from the Cameroons to Angola and eastwards to the coast opposite Zanzibar. To the east and south of the Niger Delta, Maxwell’s Duiker appears to be represented by the present species, which, as we have explained above, is of the same size and closely resembles it in most particulars. The Black-rumped Duiker was first described by Gray in an article on new species of this group, published in the ‘Annals and Magazine of Natural History’ for 1846, from specimens in the British Museum, transmitted from Fernando Po by James Thompson, one of Lord Derby’s collectors. Shortly afterwards it was figured in the ‘Knowsley Menagerie ’ by Waterhouse Hawkins from living specimens brought home by the same traveller. We will now endeavour to give some idea of the range of this still imperfectly-known species. Beginning on the north, we find skins of it in the British Museum obtained in the wooded district of Cameroons and transmitted to the British Museum by the late Captain Burton and by Crossley, besides the typical specimens received from Fernando Po, which were probably originally obtained from the adjoining mainland. Herr Matschie, in an article on the Mammals of the Cameroons, published in 1891, likewise records the occurrence of this species in the Wuté district of that country, as testified by a skull sent to Berlin by Lieut. Morgen. From Gaboon there is a skin of an adult male in the British Museum, obtained by Mr. DuChaillu, and a specimen in the Berlin Museum procured by Buchholtz. To the south of the Congo this Duiker has been obtained at Capangombé in Angola by the well-known Portuguese collector Anchieta. M. de Bocage in 1878 based a new species on these specimens and proposed to dedicate it to M. d’Anchieta, having been misled by the indifferent figure and imperfect description of _C. melanorheus_ given in the ‘Knowsley Menagerie.’ But there appears to be no reasonable doubt that they may be properly referred to _C. melanorheus_.