760 MANUAL OF ZOOLOGY. 



more or less closely allied to these occur in other countries, 

 and they are especially abundant in North America. 



In the genera Pteromys and Stiuropterus, 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 what 

 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, Siberia, and North 

 America. 



The Marmots {Ardomys), unlike the true Squirrels, are ter- 

 restrial in their habits, and live in burrows, having short tails, 

 thick bodies, and short legs. Various intermediate forms, how- 

 ever, are known, by which a transition is effected between 

 the typical Squirrels and the Marmots. Such, for example, 

 are the Ground Squirrels (Tamtas) of Europe, Asia, and North 

 America. There are numerous species of this family inhabit- 

 ing various parts of Europe and northern Asia, and generally 

 distributed over the whole of North America. Good examples 

 are the Alpine Marmot (A. Alpinus) of Europe, and the 

 Prairie Dog (Cynomys Ludovicianus) of North America. 



The Pouched Marmots (Spermophilus] have cheek-pouches, 

 and are widely distributed over North America, northern 

 Europe, and northern Asia. 



As regards the distribution of the Rodentia in time, very 

 many fossil forms are known, the oldest appearing in the 



Eocene Tertiary, but the ex- 

 tinct forms offer few points 

 of special interest. The fam- 

 ilies of the Stiuridce, Muridce, 

 My oxides, and Octodontidce (?), 

 have representatives in the 

 Eocene, and the families of 

 the Dipodida, Castorid(E,Hys- 

 triridce, Cavidcz (?), and Lago- 



posits. The Leporida do not 

 seem to have made their appearance earlier than the Pliocene. 

 Amongst the fossil Rodents perhaps the most interesting are 

 the extinct genera of Beavers. Of these the genera Steneofiber 

 and Palaocastor are Miocene; Chalicomys is Pliocene; the 



