286 The Rodentia. 



THE RODENTIA. 



The Rodentia is the largest order of the Mammalia in 

 the number of species and individuals, and also the most 

 widely distributed. Most of the rodents are land animals, 

 and a large proportion of them live in colonies in subter- 

 ranean burrows of their own construction; some, however, 

 like the Beaver, Water Vole, Nutria and Muskrat, are more 

 or less aquatic, and others like the Squirrels lead arboreal 

 lives. In speaking of distinguishing characteristics, 

 Lydecker says : 



"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 inthe hares and rabbits and their 

 allies a second smaller pair occur behind those of the up- 

 per 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 distiaguished by the presence of a long gap be- 

 tween 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 corre- 

 sponding 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 mentioned. The feet are usually fur- 

 nished with five toes, which generally terminate in sharp 



