SCIURID& 453 



true Squirrels, from which they differ in several unimportant 

 details. There is only one functional premolar the small anterior 

 one usually found in Sciurus being either absent altogether or quite 

 small and functionless. There are some four well-defined species, 

 all found in North America, one (T. asiaticus) extending also through 

 Siberia into Eastern Europe. 1 They are generally known as Ground- 

 Squirrels, but in America, where they are among the commonest 

 and best known of the indigenous Rodents, as " Chipmunks." The 

 members of this genus seem to lead into the genus Spermophihts, 

 so that the division of the Sciuridce into two subfamilies, although 

 convenient for classification, is rather artificial. 



Remains of Tamias, probably belonging to existing species, occur 

 in the Pleistocene deposits of Europe and Nebraska. 



Pteromys 2 and Sciuropterus. 3 The Flying Squirrels, although in- 

 capable of true flight, can yet float through the air for considerable 

 distances by the aid of an extension of skin connecting their fore 

 and hind limbs, and forming a sort of parachute. This parachute 

 is merely a lateral extension of the ordinary skin of the body, 

 which passes outwards between the limbs and terminates at the 

 wrists and ankles. In addition to the lateral membrane there is a 

 narrow and inconspicuous one passing from the cheek along the 

 front of the shoulder to the front of the wrist, and another at 

 least in the larger species stretching across behind the body from 

 ankle to ankle and involving the base of the tail. The Flying 

 Squirrels are divided into three genera. Of those with a normal 

 dentition Pteromys contains the larger and Sciuropterus the smaller 

 species. The two differ in certain details of dentition, as well as 

 in the greater development in the former of the expanded mem- 

 branes, especially of the " interfemoral " or posterior membrane, 

 which in the latter is almost wholly absent. In Pteromys the tail 

 is cylindrical and comparatively thin, while in Sciuropterus it is 

 broad, flat, and laterally expanded, and evidently compensates for 

 the absence of the interfemoral membrane by acting as a supple- 

 mentary parachute. In appearance Flying Squirrels resemble the 

 other forms, although they are even more beautifully coloured. 

 Their habits, food, etc., are also very similar to those of the 

 true Squirrels, except that they are more decidedly nocturnal, 

 and are therefore less often seen by the traveller ; their peculiar 

 shrill cry is, however, well known to all who have camped out in 

 the regions which they inhabit. Their mode of flight is precisely 

 similar to that of the Flying Phalangers of Australia. Of each of 

 the two genera there are about thirteen or fourteen species, all 



1 Some American zoologists have recently proposed to raise a large number of 

 the forms usually regarded as local races to the rank of species. 



2 Cuvier, Lemons d' Anatomic Comp. (1800). 



3 Cuvier, Ann. du Museum, vol. x. p. 126 (1825). 



