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ORDER X. 

 THE RODENTIA. 



THE Rodentia are so named, from the structure of 

 the incisor teeth, which, to the number of two in 

 each jaw, give them the power, and necessitate the 

 practice of gnawing, for they grow up from the roots 

 with such rapidity, that, when not sufficiently used 

 upon hard substances, they rise so that they cannot 

 act upon each other, and the animal perishes of 

 hunger. Behind the incisors is a void space, there 

 being no canines, and the molars are either with 

 flat crowns, or tuberculated. The cranium is very 

 flat, the orbits not separated by temporal fossae ; the 

 brain almost without convolutions, though rather 

 more complex than in Edentata, approximates the 

 order in more respects to the Marsupials, and, in 

 particular, by the additional character of the teeth, 

 with the genus PHASCOLOMYS ; the eyes are directed 

 sideways, and their organisation, perhaps with some 

 exception of the Edentata, is in general inferior to 

 the preceding. 



So late as the publication of the " Mammalogie* 

 of Desmarets, 1822, the genera of Rodents were only 

 24, making 144 species ; Mr. Waterhouse, however, 

 in 1839, enumerates no less than 58 ; and new 



