546 MANUAL OF ZOOLOGY. 



other countries, and they are especially abundant in North 

 America. 



In the genus Pteromys, or Flying Squirrels, there is a peculiar 

 modification by which the animal can take extended leaps 

 from tree to tree. The skin, namely, extends in the form of a 

 broad membrane between the hind and fore legs, and this acts 

 as a kind of parachute, supporting the animal in the air. There 

 is, however, no power whatever of true flight, and the structure 

 is identically the same as wriat we have previously seen in the 

 Flying Phalangers (Petaurus\ which take the place of the 

 Flying Squirrels on the Australian continent. The Flying 

 Squirrels are found in southern Asia, Polynesia, the north-east 

 of Europe, and Siberia. 



The Marmots (Arctomys), unlike the true Squirrels, are ter- 

 restrial in their habits, and live in burrows. Various inter- 

 mediate forms, however, are known by which a transition is 

 effected between the typical Squirrels and the Marmots. There 

 are numerous species of this family inhabiting various parts of 

 Europe and northern Asia, and generally distributed over the 

 whole of North America. 



CHAPTER LXXXI. 

 CHEIROPTERA. 



ORDER XL CHEIROPTERA. This order is undoubtedly " the 

 most distinctly circumscribed and natural group " in the whole 

 class of the Mammalia. In many respects, however, it would 

 be advantageous to regard the Cheiroptera as a sub-order of 

 the next order (namely the Insectivora) specially modified to 

 lead an aerial life ; just as the Pinnigrada are regarded as a 

 mere section of the Carnivora specially modified to suit an 

 aquatic life. 



The Cheiroptera are essentially characterised by the fact that 

 the anterior limbs are longer than the posterior, the digits of 

 the fore-limb, with the exception of the pollex, being enor- 

 mously elongated (fig. 209). These elongated fingers are 

 united by an expanded membrane or " patagium," which is 

 also extended between the fore and hind limbs and the sides 

 of the body, and in many cases passes also between the hind- 

 limbs and the tail. The patagium thus formed is naked, 

 or nearly so, on both sides, and it serves for flight. Of the 



