276 



ANATOMY OF VERTEBRATES 



153 



with their low position in the mammalian scale the prevalent 

 habit amongst the Marsupialia of limiting the exercise of the 

 faculties of active life to the period when they are shielded by 

 the obscurity of night. 



The Lissencephala or smooth-brained Placentals form a group, 



equivalent to the Lyencephala or 

 Implacentals, and include the fol- 

 lowing orders, Rodentia, Insecti- 

 vora, Cheiroptera^ and Bruta. The 

 RoDENTiA are characterised by 

 two large and long curved incisors 

 in each jaw, fig. 153, z, separated by 

 a wide interval from the molars ; 

 and these teeth are so constructed, and the jaw is so articulated, 

 as to serve in the reduction of the food to small particles by 



154 



Skull of a Rodent (Jerhoa). 



Pteromys Volucella. 



acts of rapid and continued gnawing, whence the name of the 

 order. The orbits, ib. o, are not separated from the tem- 

 poral fossas. The testes pass periodically from the abdomen 



