6 Bailf)/ — Identity of Thomomys umbrinus (Richardson). 



the most accessible localities in Southern Mexico from which 

 any species of Thomomys is known. 



The following description is based on 8 specimens from Boca 

 del Monte, Vera Cruz, Mexico: 



Thomomys umbrinus (Richardson). 



Geomys umbrinus Richardson. Fauna BoreaU-Americana, I, pp. 190 and 

 202, 1829. 



General characters. — Size medium, hind foot 27 to 28; colors dichromatic, 

 dull umber brown or plumbeous black ; skull short and wide with strongly 

 projecting incisors. 



Color. — Upperparts, in brown phase, dull burnt umber or Prouts brown, 

 varj'ing to nearly black in some specimens; lowerparts lightly washed 

 with pale buff or whitish, in some specimens sharply contrasted with dark 

 upperparts; feet, tip of tail, chin, and sometimes throat, white. In black 

 phase slaty Ijlack all over except white chin, feet, and tip of tail. 



Skull. — Short and wide with incisors projecting well beyond nasals ; angle 

 of frontal projecting into notched anterior base of zygoma ; lachrymal 

 applied for nearly its whole length to zygoma ; premaxillse of approximately 

 the same length as nasals and widest near blunt posterior tips ; incisors 

 slender, in comi)arison with those off alms; color of incisors varying from 

 light yellow to dark orange. 



Measurements. — Average of seven adults from Boca del Monte : Total 

 length, 19.3; tail vertebrae, 58; hind foot, 27. Skull, No. 64,091, c?, from 

 Boca del Monte : Greatest length, 36 ; diastema, 13.5 ; front of occiput to tip 

 of nasals, 33 ; nasals, 12.5 ; interorbital constriction, 6.4 ; zygomatic breadth, 

 24 ; alveolar length of upper molar series, 7.5. 



