8 Field Museum of Natural History — Zoology, Vol. X. 



extended posteriorly beyond anterior root of zygoma and ending 

 about on the plane of the middle of the first upper molar; inter- 

 parietal very small. Molars very broad and more hypsodont than 

 in Mus; first upper molar longer than second and third combined; 

 second upper molar broader than long and very obliquely set in tooth- 

 row ; cusps of molars numerically as in Mus; upper incisors decidedly 

 projecting. 



Thamnomys surdaster polionops subsp. nov. 



Type from Lukenya Mountain, British East Africa. No. 17 112, 

 Field Museum of Natural History. Adult male. Collected Decem- 

 ber 20, 1905, by E. Heller. 



Characters. Similar to Thamnomys surdaster but less tawny on the 

 sides from the nose to the flanks; sides of face, arms, and sides of body 

 more or less olivaceous gray with slight mixture of tawny; a distinct 

 dusky area from the eye to the base of the whiskers; upperparts 

 bright tawny as in surdaster. Skull much as in surdaster, but averag- 

 ing slightly larger and broader. Distinguished from Thamnomys 

 dryas by the possession of 6 mammae instead of 4; from T. mac- 

 millani by larger size and darker color; from T. ibeanus by smaller 

 size, smaller ears, brighter color, and paler under sides of hind legs. 



Measurements. Type: Total length 284; head and body 120; 

 tail vertebrae 164; hind foot (s. u.) 24; ear 16. Skull of type: Great- 

 est length 30.4; basilar length 22.9; zygomatic breadth 14.3 ; nasals 

 11; least interorbital breadth 4.5; postpalatal length 10.5 ; diastema 

 7.4; palatine slits 6.8; maxillary toothrow 4.5. 



Thamnomys ibeanus sp. nov. 



Type 'from Molo, British East Africa. No. 17096, Field Museum 

 of Natural History. Adult male. Collected March 15, 1906, by 

 E. Heller. 



Characters. Size large; color dark and relatively dull; hind legs 

 just above tarsal joint usually dark-colored all around; skull and 

 teeth relatively large. Most similar to Thamnomys dolichurus but 

 color darker; size larger; teeth sHghtly larger ; audital bullae smaller. 

 Somewhat similar to T. surdaster and T. s. polionops but size larger 

 and color duller; skull and teeth larger; audital bullae relatively 

 small, about equalling those of polionops in actual size. 



Measurements. Type: Total length 329; head and body 136; 



tail vertebrae 193; hind foot (s. u.) 26; ear 21. Average of 8 adults 



from Molo, Gilgil, and Lake Elementeita: Total length 305 (290-330) ; 



. head and body 121 (109-136); tail vertebras 184 (174-204); hind foot 



