620 



THE PRIMATES 



xxiii. 7- 



anthropoids (Fig. 383). The incisors become spatulate, rather than 

 pointed, and the premolars bicuspid (Figs. 396, 397). 



The reproduction is characterized by the presence of menstrual 

 cycles, continuing throughout the year. Ovulation occurs once in each 

 cycle, often accompanied by the development of sexual signals and 

 behaviour patterns by the female. There is a discoidal, haemochorial 

 placenta, with very early development of the extra-embryonic meso- 

 derm and reduction of the yolk-sac, amniotic folds, and allantois. 

 Usually a single offspring is produced and there is one pair of pectoral 

 mammae. The young are looked after for a long period. The social 

 life is often based upon families of one male and several females and 

 young. 



Fig. 398. Spider monkey, Ateles. (From life.) 



8. New World monkeys, Ceboidea 



The continent of South America houses a special type of monkeys, 



as of so many other mammalian groups. These platyrrhine (flat-nosed) 



monkeys have presumably been isolated since Eocene times. They 



could not have been a later immigration from North America because, 



so far as we know, no cercopithecoids or hominoids reached that 



continent until man came. The differences from the Old World 



monkeys are not very profound, however; therefore either the 



characteristic monkey organization had appeared in the Eocene or the 



platyrrhines and catarrhines have evolved on parallel lines. 



2.1.3.3 . . 



In the teeth the second premolar is retained ( ), whereas it is 



2.1.3.3 



lost in all Old World forms; the molars are quadritubercular (Figs. 



383 and 396). The brain is relatively larger in marmosets even than 



in man, but this results from the small size of the animal. The smaller 



species show little Assuring, but this develops in the larger ones (Ateles), 



showing a pattern similar to that of Old World monkeys. The nasal 



