EVOLUTION OF THE PRIMATES AND MAN 461 



The tail of a Lemur is long but not prehensile, and its skull 

 may be recognised by the fact that the cavity of the orbit can 

 still communicate with that of the temporal fossa beneath the 

 postorbital bar. 



Another Eocene fossil allied to Notharctus is Tetonius, 

 the earliest representative of the group Tarsioidea. Tetonius 

 had an enlarged rounded brain-case and a small face. Its 

 brain must have been relatively larger than in any other 

 known Eocene animal. It also was not on the direct line 

 of descent owing to specialisations such as the loss of the 

 lower incisors, but a close relative of it must have been the 

 ancestor of Tarsius which lives at the present day. In Tarsius 

 the postorbital bar is splayed out and almost but not quite 

 prevents communication between the orbit and the temporal 

 fossa. It shows important advances in the structure of the 

 brain, and of the external ear. In the fact that it has a discoidal 

 placenta with a thickened trophoblast hollowed out into 

 lacunae filled with maternal blood, and in the fact that the 

 mesoderm appears very early in the development of the 

 embryo, Tarsius resembles the higher Primates and Man, and 

 differs from the Lemuroidea. 



In the true monkeys, apes and man, or Anthropoidea, the 

 orbit is completely shut off from the temporal fossa. From 

 some Tarsioid ancestor with affinities to Notharctus there 

 diverged a branch which gave rise to the New World monkeys. 

 These forms which are included in the group Platyrrhinas have 

 a broad internasal septum with the nostrils wide apart, and a 

 tail which is usually prehensile. They show a considerable 

 advance in the structure of the brain, and in the fact that their 

 dental formula is reduced to i§, c\, p|, m-?. At the same time 

 they are definitely off the main line of Primate evolution 

 because of the structure of the tympanic bone which forms a 

 ring. 



The Old World monkeys, apes and man, form the group 

 Catarrhinae, in which the internasal septum is narrow, the 

 tympanic bone forms a tubular external ear, the tail when 

 present is never prehensile, and the dental formula is reduced 

 to if, cf, p|, inf. 



