The dentition of the _Cebidæ_ is very characteristic of the family. The dental formula is I2/2, C1/1, P3/3, M3/3 and the teeth thirty-six in all--a larger number than is found in any of the Old World forms, or in the species of the last family (the _Hapalidæ_); for they possess an extra pre-molar tooth above and below on each side. Their molar teeth are four-cusped; and in the upper molars of the Spider-Monkeys (_Ateles_) and of the Howlers (_Alouatta_) there is an oblique cusp, such as is found in the molars of the _Lemuroidea_, joining the hind inner to the front outer cusp. Among the _Cebidæ_ the brain varies {152}very much; the posterior lobes of the main brain (or cerebral hemispheres, which are almost always convoluted) are also almost always so large as to entirely cover over the cerebellum (or hind brain), a relation which does not exist between these two regions of the brain in the _Lemuroidea_. The cerebellum, however, in the Howlers is slightly uncovered. The absolute size of the brain never, in any Ape, approaches that of Man. None of the _Cebidæ_ attain the size of even the medium-sized Old World Apes. The _Cebidæ_ are all arboreal, and strictly confined to the forest regions of Tropical America, from the southern part of Mexico to about the parallel of 30° S. lat. They are divided into four sub-families, namely: The Douroucolis, or Night-Monkeys (_Nyctipithecinæ_); the Saki Monkeys (_Pitheciinæ_); the Howlers (_Mycetinæ_); and the Capuchin Monkeys (_Cebinæ_). THE NIGHT-MONKEYS. SUB-FAMILY NYCTIPITHECINÆ. The Night-Monkeys are small and elegant animals covered with long hair, and having long bushy tails, which are not prehensile, although they can be curled round a branch of a tree. The caudal vertebræ in these creatures are consequently not flattened from above downward, as is the case in the prehensile-tailed groups, but rounded. Their lower incisor teeth are set vertically and their thumb is fairly well developed. This sub-family contains three genera, the Squirrel-Monkeys (_Chrysothrix_); the Whaiapu-Sais, or Titis (_Callithrix_); and the typical Night-Monkeys, or Douroucolis (_Nyctipithecus_). THE SQUIRREL-MONKEYS. GENUS CHRYSOTHRIX. _Chrysothrix_, Kaup., Thierreich., i., p. 51 (1835). {153}The Saimiris, or Squirrel-Monkeys, are very beautiful and active little animals, characterised by their soft, close, and erect fur, and especially in having the head produced posteriorly. The face is relatively small and has a high facial angle. The eyes are large, directed forwards, and set very close together. The ears are large; and the nose has a very broad partition between its nostrils. The tail is long, round, and covered with short hair; but tufted at the extremity and non-prehensile.