22 THE DESEADO FORMATION OF PATAGONIA 



would seem then that the country for some reason was less 

 adapted to edentates, and that in some other part of 

 South America they were flourishing and evolving. 



In the Deseado the rodents appear for the first time in 

 South America. They are all Hystricomorpha and in a 

 relatively primitive stage of development, but they are 

 typically developed already. Did they migrate in from 

 some other locality, or were they evolved on the spot? 

 Ameghino believed that they were developed from some 

 such form as Promysops or Propolymastodon of the Casa- 

 mayor, and that these forms were ancestral to rodents all 

 over the world. If my interpretation of the age of these 

 beds is anywhere near correct, this last at least is impos- 

 sible, for in North America and Europe typical rodents 

 are present in the Eocene. Then as to even the hystri- 

 comorphs being developed in Patagonia, I am very skep- 

 tical, for the material offered in evidence of this is very 

 insufficient, especially in the region of the incisors; and 

 may be interpreted in other more probable ways. I am 

 confident that either just before the beginning of the Des- 

 eado, or at the beginning, the rodents of these beds mi- 

 grated, either from some other continent, or at least from 

 some other section of South America into this Patagonian 

 region. 



Some idea of the type of country and the climate of the 

 Deseado period in Patagonia may be obtained by ana- 

 lyzing the fauna as to the character of its teeth as indicative 

 of the food; and by studying the feet as indicative of the 

 ground on which they were used. 



The Typotheria with their chisel-like front teeth, lack 

 of canines, and their permanently growing grinders evi- 

 dently ate a hard type of vegetation. Deep and permanently 

 growing molars are characteristic of the eaters of grass, 

 a form of vegetation which is especially hard on the grinding 

 teeth, on account of the silica in the stems and leaves. 

 This however would scarcely necessitate the development 



