4 Bailey Identity of Thomomys umbrinus (Richardson). 



sides ; a white patch on chin and throat ; tail as in fossor, but a little darker, 

 apparently dirty, not bicolor as infulvus ; feet dull whitish. 



Skull nearer that of fulvus than of fossor or lachuguilla, but smaller, the 

 rostrum shorter and relatively broader, nasals and premaxillee ending in 

 line with each other ; anterior base of zygoma, viewed from above, emargi- 

 nate instead of rounded, lachrymal applied almost' entirely to zygoma in 

 stead of frontal. [Description accompanied by rough drawing showing 

 peculiarity of zygoma.] 



Measurements. Skin, measured dry : Total length 220 ; tail 45 ; hind foot 

 2fi. Skull : Greatest length 37.6 ; diastema 14 ; front of occiput to tip of 

 nasals 34 ; nasals 13 ; interorbital constriction 6.4 ; zygomatic breadth 

 (approximately) 26 ; upper tooth row (alveola) 7.8. 



From this description it was evident that the type did not 

 agree with any species of Thomomys from the United States, but 

 that it did agree closely with an unrecognized species in the 

 Biological Survey Collection, from Boca del Monte, Vera Cruz, 

 Mexico. After Mr. Miller's return, one of the Boca del Monte 

 specimens was sent to Mr. Oldfield Thomas, Curator of Mam 

 mals in the British Museum, for comparison with the type. 

 Mr. Thomas kindly made the comparison and was not convinced 

 that they were the same ; but the discrepancies which he pointed 

 out, namely, broad and strongly orange-colored incisors, longer 

 nasals, broader posterior tip of premaxillaB, larger size, and 

 much stronger color of the type specimen, are practically 

 covered by individual variation in the series from Boca del 

 Monte. The full size photograph of the skull of the type, fur 

 nished by Mr. Thomas and here reproduced, shows unmistakable 

 characters, restricting umbrinus to a group of forms occurring only 

 in southern Mexico. The strongly emarginate, instead of 

 rounded, anterior base of the zygoma does not occur in any species 

 of Thomomys from the United States. The combination of this 

 character with a short wide skull, projecting incisors, wide pos 

 terior part of premaxillse, and the peculiar position of the 

 lachrymal which lies almost entirely against the jugal instead 

 of mainly against the frontal, occurs only in orizabas, peregrinus 

 and the Boca del Monte form. External characters which still 

 further restrict the name umbrinus to the Boca del Monte form 

 are the white throat and light lowerparts, in strong contrast to the 

 dark upperparts. Boca del Monte also has the advantage of 

 being the farthest east and probably, previous to 1829, one of 



