The Squirrel Monkeys, genus Chrysothrix, are small creatures with a long head, the occiput projecting. Their tail, though long, has no naked area at the extremity and is non-prehensile. It is a remarkable fact that the proportions of the cranium as compared with the face are greater, not only than in other Monkeys, but than in Man himself. The thumb is short, but not so short as in the Spider Monkeys. The cerebral hemispheres are very smooth; but, as already remarked, this is a matter of size, and not of low position in the series. It may appear at first sight that this statement contradicts the one made concerning the Howlers. But the latter are large Monkeys, and therefore ought, so to speak, to have a more complex brain; but they have not. Like so many of the American Monkeys, the Squirrel Monkeys are gregarious, and, in spite of their tails, arboreal. They are largely insect-feeders, and also catch small birds and devour eggs. There are four species, of which C. sciurea is the commonest, and is constantly an inmate of the Zoological Society's Gardens. Humboldt asserted of it that when vexed its eyes filled with tears; but Darwin did not succeed in seeing this very human expression of an emotion.
Callithrix is a genus not far removed from the last, and, like it, occurs both in Central and in South America. It is chiefly to be distinguished from Chrysothrix by the non-extension backwards of the head, and by the more furry character of the tail. The lower jaw is rather deep, as in the Howlers; but there is not, or there has not been discovered, a howling apparatus like
that of Mycetes. Nevertheless Professor Weldon[[416]] has found in a female of C. gigot a patch of ossification on the thyroid cartilage of the larynx which may be an indication of something more in the male. There are eleven species.
Nyctipithecus, the Doroucouli Monkeys, is a genus of somewhat Lemurine appearance, caused by their large eyes. But they reminded Bates of an Owl or a Tiger-cat! They have a long, but not prehensile tail. As in the Marmosets, the lower incisors project forwards in a Lemurine fashion. The thumb is very short. A peculiarity of this genus is the twenty-two dorso-lumbar vertebrae. As in Chrysothrix, but not as in Callithrix, the hemispheres of the brain are smooth. There are five species, of which one occurs so far north as Nicaragua; the rest are Brazilian, extending down to the Argentine.
Fig. 266.—Red-faced Ouakari. Brachyurus rubicundus. × 1⁄5.
The Ouakari Monkeys, Brachyurus,[[417]] are, as the name denotes, short-tailed forms. Two species, B. rubicundus and B. calvus, have bright red faces; B. melanocephalus has a black one. There is a small thumb. The brain is fairly convoluted, and is to be specially compared with that of Cebus and Pithecia. The
species B. rubicundus at any rate has an absolutely as well as a relatively greater length of intestines and caecum than any other American Monkey known.
Fig. 267.—White-nosed Saki. Pithecia albinasa. × 1⁄5. (From Nature.)