304 



MICROTUS. 



Measurements. Total length, 124-130; tail vertebrae, 16-18; 

 hind foot, 16.5-17; ear, 13-14. Skull: occipito-nasal length, 24.5; 

 Henset, 22; zygomatic width, 19; interorbital constriction, 3.5; length 

 of nasals, 7.5 ; palatal length, 11.5 ; length of upper tooth row, 6. 



0. Orthriomys. 



First lower molar with one external and two internal closed 

 triangles, and two open triangles; third lower molar with one external 

 and one internal closed triangle and two internal transverse loops; 

 one reentrant angle between the two closed triangles; third upper 

 molar with one external and one internal closed triangle; anterior 

 loop in second and third upper molars pyriform. 



FIG. 53. MICROTUS (ORTHRIOMYS) UMBROSUS. 



No. 68508 U. S. Nat. Mus. Coll. Twice nat. size. 



UPPER TOOTH ROW. LOWER TOOTH ROW. 



Enlarged 7 times. Enlarged 7 times 



