" Mu3 " iiigiicaiula, Thos., and woosnanii, Schtcann. 141 



Woo'snani, so that we are now enabled to make a better 

 study of tlie animal. In addition, excellent notes on the 

 haljits have been made bv Mr. Heller, who obtained in East 

 Africa his " Thamnoviys lorinffi" a form undoubtedly — as 

 Mr. flollister has shown * — very closely allied to nigricnuda. 



On usinij; my key to the subgenera, one finds that it is with 

 yEtltoinys alone that niyricauda needs comparison, and on 

 making this I come to the conclusion that its specializations 

 for an arboreal life are, undoubtedly, of sufficient ira- 

 ])ortance to render it Avorthy of superspecific distinction. 

 Moreover, since there is complete discontinuity, I think it 

 most convenient to make a genus for it, rather than a sub- 

 genus of Rattus. 



This may be called : — 



Thallomys, gen. nov. 



Genotype, Thallomys niyricauda [Miis nigricauda, Thos.). 



Other forms described : loringi,Yie\\.', kalaharicus, DoWm. 



External form modified in the way usual in arboreal forms, 

 i. e. with the feet comparatively shortened, with large pads 

 and comparatively long fifth digits, and with the tail pro- 

 fusely pencilled throughout, quite different from the nearly 

 naked tail of ^thorny s and other terrestrial rats, while even 

 the blackish line through the eyes so characteristic of many 

 arboreal rodents is here again present. Mammae — 2 = 4. 



Skull essentially as in ^thomys, the bullse unusually 

 large. 



Upper molars with the cusps high and well marked, the 

 valleys on each side of the middle row of cusps deep and well 

 defined, and the middle cusps themselves markedly narrower 

 and more prominent than in JEthomys, i. e. nearly circular 

 instead of transversely oblong. 



Lower molars with an approach to that peculiar condition 

 which is found at its maximum in Mylomys and certain other 

 genera, the cusps high and very sharply defined, their wear- 

 ing surfaces pointing forwards, and the median valley along 

 the tooth-row very sharp and deep. Almost no trace present 

 of median posterior supplementary cusps. 



These characters, and especially those of the lower molars, 

 seem to justify the generic distinction of the group, while 

 the hairy tail separates it from its allies in exactly the same 

 Avay, and for the same reasons, as Nyctomys and lihipidomys 

 are distinguished in America from other Vesper-rats, and in 



♦ liull. U.S. Nal. Musi. uo. \)i\ p. 00 (1919). 



