CHAPTER XXXI. 



The Rodents, — Order Rodextia. 



Squirrels, Marmots, and Beavers. 



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

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

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

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

 .Mammalian 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 arc 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 protection 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 leans. 



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

 characterised 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 occur 

 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 upper 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 exception of the aye-aye among the lemurs. 



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



vol. in. — 5 



