50 FIELD MUSEUM or NATURAL HISTORY ZOOLOGY, 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 dus'ky above, 

 dull whitish below. Mammas 3-3 = 6. 



Skull similar to that of 0. 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 vertebrae 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 0. dry as humilior, it shows greater 

 resemblance to 0. vegetus of Panama, especially in the extent of white 

 on the throat and in the forwardly projecting zygomatic plate. 0. 

 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. 



