A Hand-book to the Primates, Volume 1 (of 2)

by Henry O. Forbes

The dentition of the Cebidæ is very characteristic of the family. The dental formula is I22, C11, P33, M33 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 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).

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.

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 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.