THE PAMPEAN FAUNA 291 



representatives of northern groups seem more ancient than they really 

 are. Conversely, northern representatives of southern groups will be 

 more primitive than their contemporaries in the south, and in certain 

 correlations the comparison of the two may furnish the mean homotaxial 

 relation representing actual synchronism. 



The Pampean Fauna: Equid^ and Ground Sloths 



Equus, for instance, appears first in the Upper Pliocene in the Old 

 World, Lower Pleistocene in North America, Middle Pampean in Argen- 

 tina. It is of Holarctic, probably Palearctic origin. In South America 

 it is accompanied and partly preceded by peculiarly modified and some- 

 what less progressive genera, Hippidion and Onohippidion, in which in- 

 fluence of North America predecessors is apparent; but the modification 

 seems to be partly due to South American environment as well. We 

 would conclude that the Middle Pampean is later than the base of the 

 Pleistocene, but how much later would be difficult to say. 



On the other hand, the Ground Sloths are of South American origin. 

 The genus Mylodon appears in the Monte Hermoso beds, below the Pam- 

 pean in South America, in North America in the Lower Pleistocene, and 

 is accompanied here by Megalonyx, less progressive but modified pecu- 

 liarly, appearing somewhat earlier, and bearing almost the same relations 

 to Mylodon. that Hippidion does to Equus. Megalonyx also appears ear- 

 lier in North America (Blanco or Middle Pliocene) and is unknown in 

 South America, although evidently descended from the Miocene Mega- 

 lonychidae, just as Hippidion is from the Miocene Pliohippiis of North 

 America. From this we would conclude that the Monte Hermoso beds 

 underlying the Pampean are older than our Lower Pleistocene. By com- 

 paring this result with that derived from the Equidae the equivalence of 

 the Pampean with the Pleistocene seems assured. 



A similar method of treatment of the various groups common to the 

 Pampean and Pleistocene fauna of South America and Eolarctica yields 

 confirmatory results. These are: 



Canidae, Ursidae, Felida?, Procyonidae, Cervidae, Camelidae, Equidae, 

 Tapiridae, Mastodontida?, Muridae, etcetera, of Holarctic origin. 



Erethizontidae, Mylodontidae, Megalonychidae, Glyptodontidae, Tatu- 

 Biidae, etcetera, of Neotropical origin, 



I Lis nol often thai so fair an opportunity as Ibis is afforded for corre- 

 lation in far distant regions. More commonly the isolation of the re- 

 gion has prevented or impeded fauna! interchange; the migration has 



