EODENTIA. 



815 



derived as a whole. This division may have been the suborder Tillodonta 

 of the Eocenes, or the Rodentia may be the descendants of the Marsupialia 

 with or without the intervention of that group. 



The differentiation of the suborders of the Rodentia evidently dates 

 from a period at least as early as the lowest Miocene. It is an important 

 fact that the Lower Eocene (Wasatch epoch) has as yet produced nothing 

 but the lowest type (Sciuromorpha). It is also true that the Puerco Eocene 

 epoch has, in sixty species of Mammalia, disclosed no Rodentia at all, while 

 Tillodonta and Tseniodonta are abundant. 



The Myomorpha first appear in the White River beds (Oligocene), but 

 none with prismatic teeth occur below the John Day epoch. The Lago- 

 morpha, on the other hand, present us with almost all their special charac- 

 ters at once, in the White River. The Hystricomorpha, whose home is in 

 South America, are unknown in North America below the Loup Fork or 

 highest Miocene, where Leidy identified a true porcupine, Hystricops 

 venustus* 



Many of the above genera stand in evident genetic connection with 

 existing forms. The Miocene Castors doubtless include the ancestor of the 

 modern beaver. The Ischyromys is a primitive type of the Sciuridce, and 

 Gymnoptyclms connects it directly with the existing forms by the character 

 of its molar teeth. Eumys is the primitive form of Hesperomys, as Paciculus 

 is of Sigmodon. Entoptychns and Pleurolicus are the near ancestors of the 

 Geomyidce of the Pliocene and present periods. Palceolagus, Panolax, and 

 Lepus form a direct genetic line. The ancient genera all differ from their 

 modern representatives in the same way ; that is, in the greater constriction 

 of the skull just posterior to the orbits, and accompanying absence of post- 

 orbital processes. This relation may be displayed in tabular form as follows : 



' See American Naturalist 1883, p. 380, where these conclusiona 'are presented. 



