“Mus” nigricauda, Thos., and woosnami, Schwann. 141 
Woosnam, so that we are now enabled to make a better 
study of the animal. Im addition, excellent notes on the 
habits have been made by Mr. Heller, who obtained in Kast 
Africa his ‘ Thamnomys loringi,” a form undoubtedly—as 
Mr. Hollister has shown *—very closely allied to nigricauda. 
On using my key to the subgenera, one finds that it is with 
ithomys alone that nigricauda needs comparison, and on 
making this I come to the conclusion that its specializations 
for an arboreal life are, undoubtedly, of sufficient im- 
portance to render it worthy 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 nigricauda (Mus nigricauda, Thos.). 
Other forms described : loringi, Hell.; kalaharicus, Dollm. 
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 A#thomys and other terrestrial rats, while even 
the blackish line through the eyes so characteristic of many 
arboreal rodents is here again present. Mammze 0—2=4. 
Skull essentially as in Athomys, the bulle 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 Aithomys, 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 
way, and for the same reasons, as Nyctomys and Rhipidomys 
are distinguished in America from other Vesper-rats, and in 
* Bull. U.S, Nat. Mus. no. 99, p. 69 (1919). 
