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. (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, pi. i (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 (nee L.), Schl., Mus. Pays Bas, vii., p. 242 



(1876, pt.) 



Characters. Face 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 



