50 Fretp Museum or Natura History — Zo6é.oey, VoL. X. 
of black-tipped hairs producing a somewhat grizzled effect most pro- 
nounced on the forehead and sides of face where fulvous is minimized 
and gray predominates. Ground color of upper parts pale clay color 
much duller than the ochraceous or ochraceous buff of the related forms 
vegetus, tenuipes, humilior, and navus; a small preauricular tuft of ochra- 
ceous-tipped hairs usually present. Under parts mostly between clay 
color and ochraceous buff almost or quite concealing the slaty bases of the 
hairs; middle of chin and throat white or whitish to bases of hairs; 
feet white; outer sides of tarsal joints broadly dusky; tail Sanaa se above, 
dull whitish below. Mamme 3-3=6. 
Skull similar to that of O. vegetus; braincase deeper and more in- 
flated; rostrum slightly more slender; zygomatic plate well. developed, 
projecting forward farther than in related forms; palatine slits long, 
extending posteriorly to slightly beyond plane of front of first cheek tooth. 
Measurements. Average of 7 specimens: Total length 183 (173-199); 
head and body 81 (76-85); tail vertebrze 102 (96-114); hind foot (c. u.) 
21.7 (20-23). Skull of type: Greatest length 23; basilar length 17.1; 
postpalatal length 7.4; palatine slits 4.1; zygomatic breadth 12.3; 
interorbital constriction 3.6; width of zygomatic plate 2; diastema 5.7; 
maxillary toothrow 3.2. 
Remarks. This species belongs to the group of small slenderly formed 
mice to which the subgeneric name Oligoryzomys has been applied. 
Although geographically nearer to O. dryas humilior, it shows greater 
resemblance to O. vegetus of Panama, especially in the extent of white 
on the throat and in the forwardly projecting zygomatic plate. O. 
navus of northern Colombia is brighter colored and has white under 
parts. O. tenuipes of Merida, which also belongs to this group, is paler 
throughout, the head and ears, especially, being more ochraceous. 
These appear to be the only described forms with which griseolus 
requires close comparison and its grayish head is sufficient to distinguish 
it from any of them. 
Our series of thirteen specimens was obtained in and about small 
grassy swamps in clearings on the upper slopes of Paramo de Tama on 
the Venezuelan side of the Tachira River. Here they were not common 
and a line of 50 carefully placed traps seldom yielded more than two 
specimens in one night. 
Thomasomys hylophilus sp. nov. Forest Vesper Rat. 
Type from Paramo de Tama, head of Rio Tachira, Santander, 
Colombia. No. 18583 Field Museum of Natural History. Adult male. 
Collected Feb. 18, 1911, by W. H. Osgood and S. G. Jewett. 
