300 MICROTUS. 
Fig. 51. MicROTUS CALIFORNICUS HYPERYTHRUS. 
No. 10758 Field Columbian Mus. Coll. Enlarged %&. 
Tooth rows enlarged 6 times, 
KEY TO SUBGENERA. 
A. Upper third molar with three closed triangles, 
lower second molar with four closed sections PAGE 
and usually a posterior loop; lower third molar 
B. Upper third molar with two closed triangles; 
lower second molar with anterior pair of angles 
confluent; lower third molar with three trans- 
VOISE IGOPS). fo. Suisecc ale ol een he eee ee .Pitymys 303 
C. Upper third molar with two closed triangles; 
lower third molar with two closed triangles and 
two: transverse logps).. .....< uae oN Le eee Orthriomys 304 
D. Upper third molar with three closed triangles 
and one crescentic loop; lower third molar 
with two closed triangles, and two internal 
transverse. loonsiiif..0 bo '/ Us Ale wh ae ae Herpetomys 305 
A. Microtus, 
Palate normal; lower third molar usually with three closed trian- 
gles; lower first molar normally with five closed triangles and nine 
salient angles; upper third molar normally with three closed trian- 
gles and seven or eight salient angles; upper incisors not grooved; 
