186 ZOOLOGICAL GARDENS. 
and posteriorly by the limbs between which they are 
extended, and by peculiar bony processes arising from 
the feet. These expansions are not naked and mem- 
branous like those of the Bats, but are actual continua- 
tions of the skin clothed externally by a dense fur 
similar to that which invests every other part of the 
body. Neither do they serve, like the flymg membranes 
of many of the Bats, the purposes of wings; their 
functions being limited to that of a parachute, giving 
to the animal a considerable degree of buoyancy, and 
thus enabling it to take leaps of almost incredible 
extent, through which it passes with the velocity of an 
arrow. The name of Flying Squirrels is consequently 
founded on an erroneous assumption ; but it may never- 
theless be admitted as a metaphorical expression of 
their most distinguishing peculiarity. 
In this remarkable character the Flymg Squirrels of 
Siberia and North America agree with those of the 
Asiatic Islands; but the latter, or at least the best 
known species among them, differ, according to M. F. 
Cuvier, in some minute particulars of their dentition, 
The differences which he has observed and figured 
appear, however, to be little more than might be pro- 
duced by detrition of the crowns of the teeth; and we 
cannot therefore regard the genus founded by him upon 
this single consideration as by any means completely 
established. At all events we should hesitate in trans- 
ferring the name of Pteromys to the newly distinguished 
group, and adopting the new term, Sciuropterus, pro- 
posed by him for the older genus. We say older, 
inasmuch as it cannot be doubted that, in separating 
the Flying Squirrels under the former name, his brother 
Baron Cuvier had chiefly in view the northern species. 
For this reason we retain the name of Pteromys for the 
present group, whether it be restricted to the latter 
