54 



OSBORN. 



monkeys or Mesodonta, Rodentia and Ta^niodonta or ancestral 

 Edentata. A land connection with South America in the early 

 Eocene would therefore have supplied Ncogcea with the eden- 

 tates as well as the stem forms from which might have been de- 

 rived its wonderful radiation of hoofed animals, the Litopterna, 

 Typotheria and Toxodontia ; together with the remarkable radia- 

 tion of the hystricomorph or porcupine-like rodents and of two 

 families of monkeys. 



The exact zoological affinities of the oldest mammalian or 

 PyrotJieriitm fauna of South America remain to be determined. 



Fig. III. — Orders of Mammals placed in their hypothetical chief centers of 

 adaptive radiation during the Tertiary Period. 



PyrotJicriiini itself is considered by Ameghino as the source of 

 the order Proboscidia while other ungulates are beheved to be 

 related to the Hyracoidea ; upon the affinities of these forms 

 turns the problem whether South America derived the sources 

 of its great radiation from Africa or from North America. (See 

 Fig. III). 



Four streams of migration to and from Neog.ea appear to 

 have occurred ; the first established its autochthonous fauna or 

 distinctive radiation of peculiar ungulates and edentates. The 

 second related this region with Africa, via Antarctica ; this con- 

 tact, in addition to the problematical Proboscidia and Hyracoidea 



