XV THE BEAVER 467 
The caecum is disposed in a spiral. The teats are three pairs, 
non-inguinal in position. 
The size of these squirrels is 16 to 18 inches exclusive of 
the tail, which is longer. These animals can make an exceedingly 
long jump with the help of their flying membrane. Nearly eighty 
yards is the longest distance given for these aerial excursions. It 
is stated that they are able to steer themselves to some extent 
while in the air. As appears to be the case with so many 
Rodents, these animals feed largely upon beetles and other insects, 
besides bark, nuts, ete. 
The allied genus Scivropterus has a much wider range. It 
extends into the Palaearctic region and into North America, 
besides being found in India. There is here no membrane reach- 
ing to the tail. The palms and soles are furry. The caecum is 
very much shorter, and so is the large intestine. The latter, in 
S. volucella, is not more than one-third of the length of the small 
intestine. In other features there are no remarkable differences 
in structure, save that the mammae, always three pairs, may be 
inguinal. 
Of the genus Lupetaurus' but a single species is known, which 
is limited to high altitudes at Gilgit and perhaps in Thibet. — Its 
principal difference from the other genera of Flying Squirrels is that 
the molars are hypselodont instead of brachyodont. The inter- 
femoral membrane is rudimentary or wanting. The one species 
is L. cinereus. It is thought to live “on rocks, perhaps among 
precipices.” Dr. Tullberg attributes the hypselodont teeth to the 
fact that the mosses upon which it is believed to feed may have 
much sand and earth intermingled, which would naturally lead to 
a more rapid wearing away of the teeth, and hence a need for a 
good supply of dental tissue to meet this destruction. 
Fam. 3. Castoridae.—This, the third family of the Sciuro- 
morpha, contains but one genus, Castor, the Beaver, with at most 
two species, one North American, the other European. This large 
todent has small eyes and ears, as befits an aquatic animal, and the 
tail is exceedingly broad and covered with scales; the transverse 
processes of the caudal vertebrae, in order better to support the 
thick tissues lying outside them, are divided in the middle of 
the series into two. The hind-feet are much larger than the fore- 
feet, and are more webbed than in any other aquatic Rodent. 
1 Thomas, J. Asiat. Soc. Bengal, lvii. 1888, p. 256. 
