622 THE PRIMATES xxm. 8 



primitive than the remainder (Cebidae). The marmosets (Fig. 399) are 

 very small insect- and fruit-eating animals, of somewhat squirrel-like 

 appearance and habits, living in tropical South America. They have 

 a thick non-prehensile tail and there are claws on all the digits except 

 the first, allowing the animals to run up trunks they cannot grasp. 

 These are probably true claws and not secondarily modified nails. The 

 pollex is not opposable. Three premolars are present, but the molars 

 are reduced to two, a condition found in no other anthropoid. The 

 cusp-pattern is tritubercular. Unlike other anthropoids the marmosets 

 give birth to two or three young and there are signs of ancient condi- 

 tions in the placenta, where the yolk-sac becomes larger than in most 

 primates. 



Unfortunately, little is known of the evolutionary history of the New 

 World monkeys ; fossils are known only back to the Miocene of South 

 America (*Homunculus). In view of the isolation and compactness of 

 the group we may feel reasonably confident that it has evolved in- 

 dependently since its origin from Eocene tarsioids. 



