154 ALLEN’S NATURALIST’S LIBRARY. 
Guerezas (Colobus). In the length of their foot the mem- 
bers 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. (/7wx/ey.) 
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 (sw/c) 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). 
Samurs usius. Dartlett, P. 7. S.;. 1871, p. 219; , Sclaver 
P. 2.9, 1872, p. 088, fig.-of head. 
Chrysothrix ustus, Gray, Cat. Monkeys Brit. Mus., p. 53 (1870). 
Saimiri sciureus (nec L.), Schl., Mus. Pays Bas, vil, p. 242 
(1876, pt.) 
Characters.—Iace white ; head grizzled grey, minutely punc- 
tulated with black, the hairs grey with black tips; outer side 
of fore-arm grizzled grey, but in some species golden ; back 
