SCIURUS. 293 
parachute when the animal takes a leap into space, and gives it a gradual 
and easy descent. Amongst the lemurs the Gadeofithecus, the Pteromys 
in the squirrels, and the Avomalurus in another family of rodents, are 
all thus provided with the apparatus necessary to. enable them to float 
awhile in the air, for flying is scarcely the proper term for the letting- 
down easy principle of the mechanism in question. 
The flying squirrels, with which we have now to deal, are in general 
details the same as ordinary squirrels, but the skin of the flanks is 
extended between the fore and hind limbs, which, when spread out, 
stretches it into a wide parachute, increased in front by means of a bony 
spur which projects from the wrist. These animals have been sub- 
divided into the large round-tailed flying squirrels, Preromys, and the 
small flat-tailed flying squirrels, Sciwropterus. The distinction was 
primarily made by F. Cuvier on the character of the teeth, as he con- 
sidered Sciuropterus to have a less complex system of folds in the enamel 
of the molars, more like the ordinary squirrels than P¢eromys ; but 
modern research has proved that this is not a good ground for distinc- 
tion. Dr. Anderson has lately examined the dentition in eleven species 
of Pteromys and Sciuropterus, and he says: “‘ According to my observa- 
tions the form of the enamel folds in youth are essentially similar, 
consisting of a series of tubercular folds which are marked with wavy 
lines in some, and are smooth in others, but in all there is a marked 
conformity toa common type. The seemingly more complex character 
of the folds appears to depend on the extent to which the tubercular 
ridges are worn by use.” He also questions the propriety of the 
separation according to the distichous arrangement of the hairs of the 
tail. After a careful examination of the organ in nearly all the members 
of the series, he writes: “I have failed to detect that it is essentiaily 
distinctive of them—that is, that the distichous arrangement of the hairs 
is always associated with a diminutive species; but at the same time 
there can be no doubt that it is more prevalent among such.” He then 
goes on to show that the tail is bushy in seventeen species, partially 
distichous in one, and wholly so in ten, and concludes by saying: 
“YT am therefore disposed to regard the flying squirrels generally as 
constituting a well-defined generic group, the parallel of the genus 
Sciurus, which consists of an extensive series of specific forms distin- 
guished by a remarkable uniformity of structure, both in their skulls and 
skeletons, and in the formations of their soft parts.” There is a laudable 
tendency nowadays amongst mammalogists to reduce as far as possible 
the number of genera and species, and, acting on this principle, I will 
follow Dr. Anderson, and treat all the Indian flying squirrels under 
Pteromys. 
