As regards the skeleton, the skull is elongated, and the arched cranial portion prolonged backwards, the length of the base of the skull being shorter than the cerebral cavity. The facial portion of the skull is relatively smaller and the cranial larger even than in Man; this character being, however, common to all the smaller representatives of particular groups, and "obviously necessary to provide the requisite amount of brain-space." (_Mivart._) The angle of the lower jaw is narrow behind. The bony partition between the nostrils is very thin and membranaceous; and that between the large orbits is also thin and imperfect. The lower incisor teeth are vertical, and in regular series with the canines, and the latter are well developed. No Primate has the teeth placed in one uninterrupted series except Man; but there is always a small interval between each upper canine and the adjacent incisor, and between each lower canine and the adjacent pre-molar. The skeleton of the hand is one-fifth of the length of the spinal column. The wrist-bones are nine in number, the central--_os centrale_--being present as in the majority of Monkeys. In _Chrysothrix_ and in _Nyctipithecus_ also, the thumb is proportionately shorter than in any other genus, except among the Spider-Monkeys (_Ateles_), and the Old World {154}Guerezas (_Colobus_). In the length of their foot the members of this genus approximate to the proportion existing in Man; and its length, compared with that of the hand, is greater in _Chrysothrix_ than in any other group of Monkeys. The cerebral hemispheres project beyond the hind brain (_cerebellum_) to a greater relative extent than in any other mammal, namely, to one-fifth of their total length. (_Huxley._) The external surface of the cerebral hemispheres is almost as little convoluted as in the Marmosets and Tamarins, in which it is almost quite smooth, yet on the inner faces of the hemispheres the more important grooves (_sulci_) are present. The opening for the passage of the spinal cord lies nearly in the middle of the base of the skull, whereas in other genera it is situated closer to the hinder region. The Squirrel-Monkeys are entirely arboreal, and found in most of the tropical regions from Costa Rica to Brazil and Bolivia, being among the commonest of the Anthropoids of the American forests. They are diurnal, and feed chiefly on insects; but they will also often attack and devour small birds. I. THE SHORT-TAILED SQUIRREL-MONKEY. CHRYSOTHRIX USTA. _Saimiris usta_, Is. Geoffr., Arch. Mus., iv., p. 15, pl. 1 (1844). _Saimiris ustus_, Bartlett, P. Z. S., 1871, p. 219; Sclater, P. Z. S., 1872, p. 688, fig. of head. _Chrysothrix ustus_, Gray, Cat. Monkeys Brit. Mus., p. 53 (1870). _Saimiri sciureus_ (nec L.), Schl., Mus. Pays Bas, vii., p. 242 (1876, pt.)