MAMMALS THAT GNAW. 95 



between the hind-legs and the root of the tail. The whole of these flying- 

 squirrels are characterised by the complex structure of their molar teeth ; 

 and as their skulls differ considerably from those of other members of the 

 family, they must be regarded as constituting a sub-family group by them- 

 selves. 



Flying-squirrels of this group have existed since a comparatively early 

 epoch in the Tertiary period, and it is consequently impossible to affiliate 

 them with any of the genera of ordinary squirrels ; so that it is quite likely that 

 they have originated from a totally extinct genus or genera. Hence, it is 

 impossible to say whether the three genera into which they are divided have 

 all taken origin from one non-volant form, or whether the power of flight 

 has been separately evolved in each of the three generic groups. Of the 

 three genera, the one known as Sciuropterus includes the lesser flying-squir- 

 rels, all of which have the crowns of their molar teeth comparatively low, 

 and the parachute of moderate width, and not including any portion of the 

 tail. Having one representative in North America, and a second in North- 

 Eastern Europe and Siberia, the lesser flying-squirrels are mainly characteristic 

 of India and the Malayan countries. While some of the larger kinds measure 

 as much as 12 inches from the nose to the root of 'the tail, in the pigmy 

 flying-squirrel of Cochin-China and Arakan the length of the head and body 

 scarcely exceeds five inches. These squirrels collect in numbers in hollow 

 trees, where they remain in slumber during the daytime, to issue forth at 

 night for the purpose of feeding. Climbing to a coign of vantage on some 

 tree, they take their flying leaps to the bough or trunk of another at n 

 lower level, not unfrequently covering a distance of some 30 or 40 yards. 

 The length of the leap is, however, still greater among the members of the 

 next genus, reaching from 60 to nearly 80 yards. The larger flying- 

 squirrels (Pteromys) form an exclusively Asiatic group, represented by some 

 ten species, and extending from the Malayan countries as far north as Eastern 

 Tibet. In addition to their superior dimensions, these flying-squirrels are 

 distinguished from the preceding group by the greater width of the parachute 

 along the sides of the body, and the enclosure of the base of the tail in the 

 portion connecting the two hind -legs. The tail itself is, moreover, completely 

 cylindrical, instead of slightly compressed ; and the molar teeth have 

 rather taller and more complex crowns than in the lesser flying-squirrels. 

 In some of the larger species the head and body may measure as much 

 as 18 inches in length, while the tail may reach to 24 or 25 inches. 

 The last member of the sub-family is the woolly flying-squirrel (Eupetannts) 

 a large species from the neighbourhood of Gilgit, distinguished by the 

 very tall molar teeth, which have flat, instead of ridged, masticating 

 surfaces. 



Finally, the five species of pigmy squirrels (Nannosciurus), of which 

 one is West African, a second from the Philippines, while the other 

 three are Bornean, constitute another sub-family, c aracterised by cer- 

 tain peculiarities in the skull and the complex structure of the molar 

 teeth. 



An unimportant family is constituted by two North American Rodents 

 known as sewellela (Haplodon), which differ from the U 1 



Sciuridce by the absence of postorbital processes in the Pam'lv S 



skull and the rootless molars. According to Dr. Merriam, Haplodontid(K 

 they are aquatic in their habits. 



The last family of the squirrel-like group of the order is represented only 



