8 Fretp Museum oF NATURAL History—Zo6Locy, 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. 17112, 
Field Museum of Natural History. Adult male. Collected Decem- 
ber 20, 1905, by E. Heller. nos : 
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 mamme 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 vertebra 164; hind foot (s. u.) 24;-ear16. 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 slightly larger; audital bulla smaller. 
Somewhat similar to 7. surdaster and T. s. polionops but size larger 
and color duller; skull and teeth larger; audital bulle relatively 
small, about equalling those of polionops in actual size. 
Measurements. Type: Total length 329; head and body 136; 
tail vertebra 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 vertebrae 184 (174-204); hind foot 
