IV.] RODENTS. 159 



Canada. Oxycena, again, is found both in America and Europe, 

 although in a lower horizon in the former than in the latter. In a 

 second family (ProviverridcR) the typical genus Proviverra is met 



m 



FlG. 33. RIGHT UPPER MOLARS OF ArctoCVOH. 



with in the Bridger Eocene of America, and the French Oligocene; 

 while in the Arctocyonidce, in which the upper molar teeth are 

 bluntly tritubercular, Arctocyon of the lowest Eocene of Europe is 

 represented by two allied genera in the American Puerco, one of 

 which has been described as Clcenodon. 



In the rodents there are three more or less widely distributed 

 existing families restricted to Arctogaea. Of these, 

 the jerboas and their allies (DipodidtzY occur in all 

 the regions with the exception of the Malagasy, Oriental, and 

 Sonoran, although the genera from the different areas are more 

 or less markedly distinct. The other two, namely the picas or 

 tailless hares (Lagomyida) and the beavers (Castoridce) are now 

 severally represented by a single genus confined to the Holarctic 

 region. The picas date from the Oligocene of Europe, and the 

 family not improbably originated in eastern Arctogaea ; while the 

 beavers have fossil representatives in both hemispheres, with the 

 Miocene and Pliocene genus Chalicomys common to the two. 



Although members of the typical genus range into the Neogaeic 

 realm as far south as Paraguay, the squirrel family (Sciuridce) may 

 be regarded as a typical Arctogaeic one, the ground-squirrels 

 (Tamias), marmots (Arctomys) and susliks (Spermophilus} being 

 restricted to the Holarctic region, though others range over the 



1 It has been suggested by Dobson that the Dipodidcz are Hystricomorpha. 

 This, however, is disproved by Dr Scott (P. Ac. Philad. 1895, pp. 269 286), 

 who finds in the Uinta Oligocene genus Protoptychus an ancestral type of the 

 family, which thus appears to be of N. American origin. From this family are 

 probably descended the Geomyidce. 



