SCLURIDAE 453 
true Squirrels, from which they differ in several unimportant 
details. There is only one functional premolar—the small anterior 
one usually found in Sciwrus being either absent altogether or quite 
small and functionless. There are some four well-defined species, 
all found in North America, one (7. asiaticus) extending also through 
Siberia into Eastern Europe:!. 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 Spermophilus, 
so that the division of the Sciuride into two subfamilies, although 
convenient for classification, is rather artificial. 
Remains of Tams, probably belonging to existing species, occur 
in the Pleistocene deposits of Europe and Nebraska. 
Pteromys* and Sciuropterus.2—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 Sciwropterus 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, Légons d’ Anatomie Comp. (1800). : 
3 Cuvier, Ann. du Muséum, vol. x. p. 126 (1825). 
