800 



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 



generally with three closed triangles Microtus 300 



B. Upper third molar with two closed triangles; 

 lower second molar with anterior pair of angles 

 confluent; lower third molar with three trans- 

 verse loops Pitymys 303 



C. Upper third molar with two closed triangles; 

 lower third molar with two closed triangles and 



two transverse loops 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 loops 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; 



