THE PLATTEEHINI, 465 



spheres are almost smooth, but the Sylvian fissure is well 

 marked, and there is a trace of that of Rolando. On the 

 inner face of each hemisphere, the calcarine fissure is deep 

 and gives rise to a well-marked hippocampus minor within 

 the posterior comu of the lateral ventricle. The corpus 

 callosum has about a thii'd the length of the hemispheres. 

 The septum lucidum is very thick, and the precommissural 

 fibres abundant. The vermis projects beyond the lateral 

 lobes of the cerebellum, and the flocculi are large. 



2. The Platyn-hini are essentially quadrupedal and planti- 

 grade, though some, Mke the Spider Monkeys {Ateles), occa- 

 sionally assume the erect posture. They all possess tails, 

 and in some genera {e.g. Ateles) this organ becomes very 

 flexible and muscular, and the under surface of its extremity 

 is devoid of hair and highly sensitive. The tail, thus modi- 

 fied, is a powerful prehensile organ, and serves as a fifth 

 hand. The partition between the nostrils is broad and 

 separates them widely, so that the nose is remarkably wide 

 and flat, whence the name of the group. The ears are 

 rounded and bare. There are no cheek pouches, nor ischial 

 callosities, in any Platyrrhine Monkey. In most, the fore 

 limbs are shorter than the hiad limbs, but the reverse is 

 the case in the Spider Monkeys. The pollex differs less 

 from the other digits than it does in the Catarrhini. It is 

 more nearly parallel with, and in the same plane as, the 

 other digits of the manus ; and though capable of extensive 

 adduction and abduction, can hardly be said to be truly 

 opposable. The hallux is large, and susceptible of exten- 

 sive movements in abduction and adduction. 



The number of the dorso-lumbar vertebrae varies from 

 seventeen to twenty-two, the greatest number being pos- 

 sessed by -A/^2/c<ipii?iecMS, which has 22 (14 -I- 8 or 15 + 7). In 

 those forms which have prehensile tails the tenninal caudal 

 vertebrae are flattened from above downwards. The arti- 

 cular surface of the head of the humerus looks more back- 

 wards than inwards; and, not imfrequently, there is a 

 foramen above the inner condyle. The cai-pus contains 

 nine bones. The poUex is generally complete, but, in Ateles, 



2 H 



