CHAPTER XXXI 

 THE RODENTS ORDER RODENTIA 



SQUIRRELS, MARMOTS, AND BEAVERS 



UNDER the common title of Gnawing or Rodent Mammals are grouped that ex- 

 tensive assemblage of small or medium-sized species which, like rats, porcupines, 

 beavers, squirrels, and hares, are characterized by their habit of gnawing. At the 

 present day these creatures are more numerously represented than in any other Mam- 

 malian order, both as regards individuals and species; the number of the latter being 

 probably fully one thousand, while the swarms of individuals by which some species 

 are represented are too well known to need more than passing mention. Nearly all 

 the Rodents are inhabitants of the dry land, and a large proportion of them seek pro- 

 tection from enemies by living in subterranean burrows which they construct for 

 themselves, and wherein they generally associate in larger or smaller colonies. 

 Some, however, like the water vole and the beaver, are aquatic; while others, like the 

 squirrels and tree-porcupines, lead an arboreal life. A few, again, have acquired the 

 power of spurious flight through the development of folds of skin along the sides of 

 the body and limbs, by the aid of which they are enabled to take long flying leaps. 



The Rodents are some of the most easily defined of all Mammals, and are best 

 characterized by the number and nature of their teeth especially those in the front 

 of the jaws. They are distinguished by the presence in each jaw of a pair of large 

 chisel-like front or incisor teeth, which grow continuously throughout the life of 

 their owners. As a rule, no other incisor teeth save these two pairs are developed, 

 but in the hares and rabbits and their allies a second smaller pair occurs behind those 

 of the upper jaw. There are no tusks or canine teeth in either jaw, and in the 

 cheek series the number of premolars is always reduced below the normal four, very 

 generally only one of these teeth being present, while in some cases even this may 

 be wanting. In consequence of the reduced number of incisor teeth, coupled with 

 the absence of canines and the reduction in the premolars, the skull of a Rodent is 

 always distinguished by the presence of a long gap between the front and the cheek 

 teeth. Indeed, the presence in all Rodents of only a single pair of chisel-shaped 

 and permanently-growing lower incisors, opposed to a corresponding pair in the up- 

 per jaw, the total absence of canines, the long gap between the incisors and the 

 cheek-teeth, and the reduction in the number of the premolars, are of themselves 

 sufficient to distinguish the Rodent order from all other Mammals, with the excep- 

 tion of the aye-aye among the lemurs. 



Among other distinctive characteristics of the group, the following may be 

 mentioned. The feet are usually furnished with five toes, which generally ter- 

 minate in sharp claws, although they sometimes have broad nails. In walking, 

 (1238) 



