ORDER OF RODENTS. 585 
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 enamei in front causes them to wear away 
Fig. 503,—American Flying Squirrel (Sciwropterus 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 
