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.)
Characters.—Face white; head grizzled grey, minutely punctulated with black, the hairs grey with black tips; outer side of fore-arm grizzled grey, but in some species golden; back grizzled grey, washed with golden, the tips of the hairs black; tail short, thick and grey, but with the tip black.
Distribution.—The forests of Bolivia and Brazil. This is a common species, inhabiting the whole of the Peruvian Amazons, and may be met with on every stream. (E. Bartlett.)
Habits.—Arboreal and gregarious, moving about in large numbers through the forest, feeding on insects—chiefly orthoptera and spiders—small birds, and fruits.