4 Mr. 0. Thomas on the Genera and 



graduated series fornied by their anterior cheek-teeth and by 

 the low and rounded nature o£ their molar ridges. 



The genera now admitted of the smaller flying-squirrels 

 may be briefly indicated as follows : — 



A. Molar pattern consisting essentially of two transverse 



ridges converging internally towards the central part 

 of a longitudinal internal crest, thongli this structure 

 is marked in the very wrinkled-toothed forms. 

 Never as described under B. 

 a. Teeth excessively wrinkled. An outwardly pro- 

 jecting angle developed at the middle of the ex- 

 ternal edge. Postero-internal cusp well developed. 

 «*. Teeth semihypsodont. p* very large, twice the 



size of 771^ Trogopterus. 



P. Teeth brachyodont. ja* not or little larger than 

 ?n'. 

 a^. p^ well developed ; jj* rather larger than ?«\ 



Ears large Beloimjs. 



l?. p^ minute ; p* rather smaller than m\ Ears 



small Pteromysctis. 



I. Teeth not excessively wrinkled. No projecting 

 angle on outer edge. Postero-internal cusp rarely 

 developed. 

 c^. /)* generally a little larger than m\ Molar ridges 



well developed Sciuropterus. 



cP, p* decidedly smaller than 7n\ Molar ridges low 



and rounded Petaurillus. 



B. Molars each with two parallel transverse ridges rising 



into cusps laterally, so that the tooth appears evenly 

 quadricuspidate from above, and shows two subequal 

 conical cusps on its inner aspect. ^^' absent lonnjs. 



An alternative key may be based on the characters of tlie 

 premolars only, as follows : — 



A. Five cheek-teeth, p^ present. 



a. Large premolar (;;*) approximately equal to or larger 

 than first molar {m^) ; p^ standing internal to its 

 anterior angle, 



a', p* very large, twice the size of ni^ Trogopterus. 



h^. p* not or little larger than ?n^ 

 a?, p^ small but functional. 



a^. p^ more complicated, the anterior of its three 

 outer cusps highest; p^ not visible exter- 

 nally Belomys. 



b*. jo* less complicated, the antero-external cusp 



shorter ; p' generally visible externally. . . . Sciuropterus. 



¥. p^ minute, apparently not functional Pteromgscus. 



h. p* decidedly smaller than'm' ; p^ in front of its centre. Petaurillus. 



B. Four cheek-teeth, p^ absent lomys. 



But, further, the genus Sciuropterus^ even as thus restricted, 





