316 



FOUNDATIONS OF BIOLOGY 



FIG. 163. Foot postures of Mammals. 

 A, plantigrade; B, digitigrade; C, unguli- 

 grade. (From Lull, after Pander and D'Alton.) 



take long soaring leaps 

 supported by wide folds 

 of skin between the sides 

 of the body and the ex- 

 tended limbs. (Fig. 167.) 

 But the Mammals have 

 not left the air untenanted, 

 for truly volant forms are 

 represented by the Bats in 

 which the fore limbs with 

 greatly elongated fingers 

 form the framework of 

 true wings. (Fig. 168.) 



Passing below the sur- 

 face of the earth, fossorial 

 animals are found such as 

 the Woodchucks, Gophers, 

 and especially the Moles, 

 which are adapted to a 

 subterranean existence by 

 bodily modifications which 

 facilitate digging. (Fig. 

 165.) The gap between ter- 

 restrial and aquatic Mam- 

 mals is bridged by the 

 Muskrats, Beavers, Otters, 

 and Seals which are more 

 or less equally at home on 

 land and in the water. 



The truly aquatic Mam- 

 mals are the Porpoises and 

 Whales which have com- 

 pletely abandoned the 



