ORDER RODENTIA. 



419 



species of the size of the Guinea-pig. The so-called Brachymys of 

 the German Miocene appears to be generically the same. 



As Myomorpha, of which the affinities are uncertain, may be 

 mentioned Heliscomys, of the North American Miocene, in which 

 there are four lower cheek-teeth, and the lower incisors are com- 

 pressed and grooved ; and Eomys (or Omegodon), from the Quercy 



I -7 



Phosphorites, in which the cheek-teeth are Pm. -, M. —. Co/onomys, 



from the North American Miocene, also belongs to this section. 

 Family Castorid/e. — With this family we enter the Sciuro- 



Fig. 1296. 



-Palatal aspect of the right upper and lower cheek-teeth of the Beaver {Castor 

 Jiber). The tooth on the right of each figure is the premolar. 



morpha, which includes all the remaining Rodents. The Beavers 



I -7 



are natatorial Rodents, with Pm. -, M. - ; the cheek-teeth being 



1 3 



semi-rooted or rootless, with re-entering enamel-folds (fig. 1296). 



Castor is represented by the exist- 

 ing Beaver in the European Pleis- 

 tocene, and by an allied form in 

 the Pliocene of the Auvergne ; 

 and also occurs in the Miocene 

 of North America, where it has 

 been named Eucastor. There 

 has been great dispute as to the 

 affinities of the great extinct 

 Beaver (fig. 1297) of the Nor- 

 folk Forest-bed and Norwich 



Crag, but it is regarded by Mr Newton as identical with both Tro- 

 gontherium of the Russian, and Diobroticus of the French Pleisto- 





c 



Fig. 1297. — Right ramus of the mandible of 

 Trogontherium Cuvieri ; from the Forest- 

 bed. One-fourth natural size. (After Owen.) 



