SUCCESSIVE MAMMALIAN FAUNAS 283 



rodent-like animals, with a pair of chisel-shaped incisors in 

 upper and lower jaw, and a second family (fHomalodonto- 

 theriidae) contained genera which would seem to have been 

 directly ancestral to those of the Santa Cruz, but were very 

 much smaller than their successors. The very large and 

 massive fPyrotheria of the Deseado stage were represented 

 by small animals, in which the grinding teeth had two pairs 

 of conical tubercles, not yet united into transverse crests. Two 

 families of the fastrapotheres, in the broad sense, were far 

 smaller than their OUgocene and Miocene descendants. To 

 the fLitopterna are referred a number of genera, in which the 

 grinding teeth were tuberculated and had very imperfectly 

 developed crests, so as strongly to suggest the teeth of the 

 fCondylarthra. However, until something is ascertained re- 

 garding the skeleton, especially the feet, of these animals, their 

 relationships wiU remain more or less doubtful. 



It will be observed that these Casa Mayor faunas not only 

 were made up exclusively of small animals, but also that 

 they already were typically and characteristically South 

 American and bore the stamp which remained essentially 

 the same; until the successive waves of migration from the 

 north so greatly modified the composition of the Neotropical 

 fauna. The absence of rodents and monkeys and the com- 

 parative unimportance of the Edentata gave a somewhat 

 different character to these ancient faunas from those of the 

 Santa Cruz and later formations. 



5. Paleocene 



North America. — A very important discovery is one lately 

 made by American Museum parties of a formation intermediate 

 between the Wasatch and Torrejon. The interesting fauna 

 of these beds has not yet been described, but it may be re- 

 marked that it contained none of the immigrant orders. 



The vegetation of the Paleocene was already very mod- 

 ern in character, and nearly all of the common forest-trees 



