14 



SKETCH OF THE HISTORY OF MAMMALIA. 



the anterior molar of the upper jaw, where they are 

 5 — 5, which is smaller than the rest. The series of 

 molars on each side are widely separate and parallel. It 

 is in the possession of a lateral fold of skin, forming, 

 when extended, a parachute, enabling them to take long 

 sweeping leaps, that the flying squirrels are distinguish- 

 able from the ordinary group. These expansions are 



3.— Teeth of Tamias. 



fully clothed with soft fur ; and they usually project in 

 a pointed form from each wrist, being there supported by 

 a long slender osseous stylet. In some species, as the 

 one figured, this is either reduced to a mere tubercle or 

 wanting. (Fig. 5.) 



The flying squirrels are conspicuous for the rapidity 

 of their evolutions : they ascend the trees with such ve- 



