304 MICROTUS. 
in 
Measurements. Total length, 124-130; tail vertebra, 16-18: 
hind foot, 16.5—-17; ear, 13-14. Skull: occipito-nasal leneth, 24.s: 
Q) / 3714 5 s, 
Hensel, 22; zygomatic width, 19; interorbital constriction, 3.5: length 
g 
of nasals, 7.5; palatal length, 11.5; length of upper tooth row, 6. 
C. 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. 
Fic. 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 
