940 RODENTIA 



posterior border (jf premaxillary fully three-fourths greatest 

 combined breadth anteriorly. Orbit lai'ger than in the related 

 species, the postoibital process situated so far back that its 

 tip lies over a point decidedly behind middle of temporal 

 fossa instead of over middle, and its posterior border nearly 

 overhangs deepest part of postorbital constriction and anterior 

 border of squamosal. Brain-case noticeably broader in proportion 

 to its length, and foramen magnum wider relatively to its height. 

 Auditory bullte larger than in Marmota marmota, the area of 

 each bulla appearing greater than that of basioccipital and con- 

 dyles instead of about equal to it. Palate relatively narrower 

 ^josteriorly, and incisive foramina wider in proportion to their 

 length than in M. marmota. Mandible more I'obust than in the 

 related animal, e.specially in front ; sigmoid flexure longer and 

 less concave. 



Teeth essentially as in Marmota marmota as regards form, but 

 anterior surface of incisors pale yellowish instead of deep orange, 

 and cheek-teeth more robust, a character especially noticeable in 

 second and third lower molars.* 



Measurements. — Male and female from Russia (skins) : head 

 and body, 600 and 530 ; tail, 190 and 160 ; hind foot, 90 and 89. 

 For cranial measurements see Table, p. 939. 



Specimens exaviincd.—Tv/o, both from Russia. 



Family PETAURISTID^. 



1855. Ptermnyinn- Brandt, M6m. Acad. Imp. Sci., St. Petersburg, Gth ser., 



Sci. Nat., VII, p. 151. 

 1857. Sciurina Blasius, Saugethiere Deutschlands, p. 2G6 (part). 

 1879. Pteromichu Anderson, Anat. and Zool. Researches, Expeds. Yunnan, 



p. 278. 

 1891. Sciurimr Flower and Lydekker, Mammals, living and extinct, p. 450 



(part). 



Geographical distrihutlon. — Wooded portions of Northern 

 Hemisphere south to the Malay region and the southern United 

 States. In Europe confined to Scandinavia and northern Russia. 



Characters. — Like the Sciuridse, but with fore and hind limbs 

 connected by a broad fold of skin extending to wrists and ankles, 

 supported anteriorly by a stifi" cartilaginous process growing from 

 wrist, the membrane serving as a parachute by means of which 

 gliding flight is effected. 



Remarks. — The Petanristldse are so sharply difl'erentiated from 

 the Sciuridce, that it seems preferable to recognize the two groups 

 as families. Eight genera are known,f one of which is repre- 

 sented in Europe. 



* This difierence between the teeth of M. marmota and M. bobak is of 

 much importance in determining the identity of subfossil remains. 



t See Thomas, Ann. and Mag. Nat. Hist., 8th ser., i, pp. 1-8, January, 

 1908. 



