HISTORY OF THE fTOXODONTIA 479 



and drawn an animal which might readily be mistaken for 

 a curious, short-eared rabbit ; and there is every justification 

 for doing this, though the character of the fur and the form 

 of the ears are, of course, merely conjectural. Perhaps the 

 ears are too small. 



Associated with ^Pachyrukhos in the Santa Cruz stage was 

 another genus of the family, "{Hegetotherium, which, though it 

 cannot possibly have been ancestral to the former, yet serves 

 to indicate, in general terms, what the ancestor must have been. 

 This is another example of the long-continued survival of the 

 more primitive together with the more advanced and special- 

 ized form. ^Hegetotherium persisted into the Pliocene, but 

 is not known from the Pleistocene. In this genus one upper 

 and two lower incisors were already enlarged, rootless and 

 scalpriform, but none of the teeth had been lost ; it is interesting 

 to note, however, that the teeth which were lacking in ]Pachy- 

 rukhos were all very small and ready to disappear. The Santa 

 Cruz species of \Hegetotherium were considerably larger and 

 more robust animals than those of '\Pachyrukhos. 



Both of these genera were preceded by very similar, almost 

 identical forms in the Patagonian, Deseado and Astraponotus 

 stages, but the family cannot be definitely traced farther back 

 than the lower Oligocene, but it very probably arose from some 

 one of the groups, with low-crowned teeth, of the Casa Mayor 

 stage. 



The family fliiteratheriidae was, in most respects, more 

 conservative and underwent less change than either of the pre- 

 ceding groups. A persistently primitive type was the genus 

 \Protypotheriuvi, which appeared for the last time in the Plio- 

 cene of Monte Hermoso, but was much more abundant and 

 better preserved in the Santa Cruz. The animal was small 

 and had the full complement of teeth, which were arranged 

 in each jaw in a continuous series, and were fully hypsodont and 

 rootless, except incisors and canine, which were rooted. None 

 of the incisors was specially enlarged, but there was a gradual 



