HISTORY OF THE fTOXODONTIA 485 



took place in three unrelated groups of hoofed animals and 

 must have occurred independently among the Artiodactyla, 

 the Perissodactyla and the fToxodontia. By no possibility, 

 so far as we are able to comprehend the course of evolutionary 

 change, could this common characteristic have been due to 

 inheritance from a common ancestry. 



The fhomalodotheres were among the largest of Santa Cruz 

 mammals, but they were then already approaching extinction, 

 while in the Deseado stage they were more numerous and varied 

 and some of them very much larger. This is an exception 

 to the more common rule, according to which the successive 

 members of a phylum increased in stature until the maximum 

 was reached and this, in many cases, was followed by extinction. 

 The rule is, however, by no means without exceptions and 

 several have already been referred to: The largest of American 

 proboscideans was the jlniperial Elephant (Elephas fim- 

 perator) of the upper PUocene and Pleistocene and in many 

 other phyla the Pleistocene species were much larger than 

 the Recent. So with the fhomalodotheres ; they reached 

 their culmination in size and importance in the Deseado stage, 

 fewer and smaller forms sm-viving into the Santa Cruz, after 

 which the entire suborder vanished. The fanuly may be traced 

 back to the Eocene, where it is represented chiefly by a genus 

 i^Thomashuxleya) which had larger canine tusks and much 

 more brachyodont teeth, but there is no way of determining 

 when the transformation of the hoofs took place. The other 

 two famiUes (fNotostylopidse, flsotemnidae) flourished chiefly 

 or exclusively in the Eocene and were small animals still very 

 imperfectly understood. 



Suborder fPrROTHERiA. fPYROTHBRES 

 This suborder was a remarkable group, still incompletely 

 known, of elephant-like animals, which reached their culmina- 

 tion and died out in the OUgocene, their last appearance being 

 in the Deseado stage. The genus \Pyrotherium from the 



