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, | 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 typeof vegetation. Deepand 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 
