The species of this genus are at once recognised by their short tail, being the only American Monkeys in which this organ is short. The fur is short and silky; the face short, and often brightly coloured. The mammæ are situated on the breast. In the skull the lower jaw is dilated behind, and certain bones, the parietal and the malar, are in contact with each other for a more or less considerable extent on the side walls of the skull. (Cf. W. A. Forbes, P. Z. S., 1880, p. 639, figs. 5 and 6.) In Old World Monkeys this contact _never_ (except slightly in _Hylobates_) takes place. This is a useful mark for discriminating between the skulls of New and Old World Monkeys. (_Forbes._) The shortness of the tail is due, not to a reduction in the number of the vertebræ, which may be 15 to 17, but in their size. In the brain the cerebrum exhibits the more important grooves characterising the brain of the higher Apes (_Simiidæ_) well developed; the cerebellum (or hind brain) is also well developed. Thus in its general characters the brain of the Uakarís approaches most nearly to that of the genera _Cebus_ and _Pithecia_ (next to be described). By reason of its greater complication and development, it departs widely from that of the Titis (_Callithrix_) and the Squirrel-Monkeys (_Chrysothrix_). A relationship to the Howlers (_Mycetes_), suggested by the external appearance of the Uakarís and the form of their lower {175}jaw, is not borne out by their internal anatomy. The caudate lobe of the liver is very large. This character distinguishes the whole of the _Cebidæ_ from the Old World families. The Uakarís are arboreal Monkeys, very gentle and timid. The distribution of the various species is singularly restricted, each being confined to a small and particular district. I. THE BLACK-HEADED UAKARÍ. BRACHYURUS MELANOCEPHALUS. _Simia melanocephala_ (Cacajao), Humboldt, Obs. Zool., p. 317, pl. xxix. (1811). _Pithecia melanocephala_, Geoffr., Ann. Mus., xix., p. 117 (1812); Schl., Mus. Pays Bas, vii., p. 227 (1876). _Brachyurus ouakary_, Spix, Sim. et Vespert. Bras., p. 12, pl. viii. (1823). _Ouakaria spixii_, Gray, P. Z. S., 1849, p. 10, cum fig. _Ouakaria melanocephala_, Gray, Cat. Monkeys Brit. Mus., p. 62 (1870). _Brachyurus melanocephalus_, W. A. Forbes, P. Z. S., 1880, p. 645, pl. lxiii. CHARACTERS.--Head and nude face-black; back, sides, thighs, upper surface of tail, and outer and inner sides of legs more or less chestnut-brown; shoulders, arms, hands, feet, and rest of tail, black. Ears large, naked, and similar in form to those in Man.