THE RODENTS 197 



the two great suborders the ancestors of the Duplieidentata 

 possessed cheek teeth not differing greatly from those exhib- 

 ited today in Arctomys and other sciurid forms. 



In thus reviewing rapidly the various forms of teeth met with 

 in modern Rodents we note that there are different degrees of 

 specialization, the most pronounced differing considerably from 

 the typical mammalian pattern but the less pronounced ap- 

 proximating fairly well to the general type. It is also known 

 that the general type was still more approximated in the 

 Eocene Paramys. Some authors believe on both paleontological 

 and embryological grounds, however, that the Rodent dentition 

 in its ancestry does not approximate but rather differs more 

 pronouncedly than it does today from the tritubercular iuber- 

 culo-sectorial type. There undoubtedly are difficulties in the 

 way of accepting the typical mammalian molar form as ances 

 tral to that of the Rodent but these are gradually disappearing 

 as we obtain more knowledge of early members of the order. 

 Since the history of the Rodents must extend back even into 

 Cretaceous times there has been ample opportunity for modi- 

 fication of tooth forms. 



Another significant fact upon which emphasis has been laid 

 from time to time in foregoing chapters, a fact which will not 

 therefore be labored at the present, is the striking similarity of 

 dentition called forth by similarity of environment and habit in 

 advanced Rodents, in certain Marsupials notably the Wombat, 

 and in the Primate Aye-aye ( see pp. 90, 115). It is worth while 

 remembering that the two land areas, not penetrated to any 

 great extent by Rodents of which order South America is at 

 the present time the headquarters, are Australia and Mada- 

 gascar and that it is in these two regions that the Wombat 

 and the Aye-aye respectively have developed through conver- 

 gent evolution. 



In noting the several adaptations of the Rodents to a com- 

 pletely herbivorous diet we have touched upon certain features 

 which we shall find reproduced in the Ungulate orders. These 



