ORDER OF RODENT8. 



583 



condyles of the lower jaw are longitudinal, not received in spe- 

 cial glenoid sockets, but gliding freely backwards and forwards 

 in longitudinal furrows. The feet are adapted for walking 

 and climbing or burrowing, the claws being well developed. 

 A peculiarity in the incisors is that they grow out as fast as 

 they are worn down ; this is due to the fact that the pulp is 

 persistent ; the enamel in front causes them to wear away 



Fig. 503. American Flying Squirrel (Sciurapterus volucella). 



behind so that they are chisel-shaped. The species are pro- 

 lific, live mostly on vegetable food, and are of small size ; 

 the muskrat, beaver, and capybara being the largest mem- 

 bers of the group, The flying squirrels (Fig. 503) take 

 short flights by means of the expansion of the skin between 

 the fore and hind legs. The Norway lemmings are notice- 

 able for their remarkable migrations from the elevated 



