246 ON THE CLASSIFICATION OF THE SCIURIDiE. 



tooth. Just below the lioUow on the left side there is a small 

 ci'est ending above in another small tooth. The bone measures 

 20 mm. in length. (Text-fig. 29, A-0.) 



The baculum of another but tinidentitied and unlocalised species 

 of Petaurisia in the British Museum difFei-s in some well-marked 

 characters from that of P. j)hilippensis. It measures only 

 B mm. owing to the shortness of the proximal portion, which 

 does not exceed the distal portion in length. The latter has its 

 hollowed surface looking more to the left, the upper or right- 

 hand rim being moi-e, and the lower or left-hand rim less 

 elevated than in P. p]iili2yi)ensis, and its distal rim is not 

 curved over and spout-like. If the ossification of the baculum 

 proceeds backwards trom the distal to the proximal end, it is 

 possible that the difference in the length of this bone in 

 P. 'phili'ppensis and the unidentified species is a matter of age. 

 (Text-fig. 29, D, E). 



The following table shows how the genera examined may be 

 distinguished by their bacula : — 



a. Baculum witli crest runniup; from the comparatively small aud 

 simple apex backwards along the left or under side of the 

 bone. 

 6. Baculum nearly straif^ht, comparatively long and slender; 



apex bilobed, crest confined to left side Glaucomys. 



b', Baculum shorter, stouter, more curved ; apex undivided, crest 

 wholly o)' partially ventral. 

 c. A single crest curving distally up on to the left side of the 



bone to the apex, which is simple, not spatulate Si/Iopetes. 



c', A supplementary crest behind the main crest, which is en- 

 tirely ventral ; apex spatulate JPetinomys. 



a'. Baculum without lateral or ventral crest, "with expanded com- 

 plex apex. 

 d. Distal end of baculum abruptlj' upturned and ending in a 



two-headed or hammer-shaped lamina Belomys. 



(V. Distal end of baculum not abruptly upturned, the apex ex- 

 panded, more or less hollowed, and complicated with accessory 

 processes. 

 e. Apex of baculum hollowed, irregularly spatulate, with small 



accessor}' processes Petaurista. 



«'. Apex of baculum armed with uprising spiniforra processes 



on the left side JSoglaucomys. 



