312 VERTEBRATE ANIMALS. 



fly, but, like the "flying" Phalangers, they take long leaps 

 from tree to tree by means of laterally-extended folds of skin. 

 The Marmots, unlike the typical Squirrels, are ground-animals, 

 and live in burrows. An excellent example is afforded by the 

 Prairie-dog (Arctomys Ludovicianus) of North America. 



ORDER XI. CHEIROPTERA (Gr. cheir, hand; pteron, wing). 

 This order is undoubtedly one of the most natural and dis- 



FIG. 157. Skeleton of a Bat (Pteropus.) (After Owen.) 



tinctly circumscribed orders in the whole class of the Mam- 

 malia, comprising only the Bats. In many respects, however, 

 it might be well to regard the order as merely a modified 

 branch of the Insectivora, just as the Pinnigrada are regard- 

 ed as a modified offshoot of the Carnivora. The Cheiroptera 

 or Bats are essentially characterized by the fact that the fore- 

 limbs are much longer than the hind-limbs, and have several of 

 the fingers enormously elongated. These enormously length- 

 ened digits are united by an expanded leathery membrane or 

 " patagium," which not only stretches between the fingers, 

 but is also extended between the fore and hind limbs, and is 

 attached to the sides of the body (Fig. 157). The patagium 



