300 
MICROTUS. 
ise 
ID 
Fic. 51. MICROTUS CALIFORNICUS HYPERYTHRUS. 
No. 10758 Field Columbian Mus, Coll. Enlarged %. 
Tooth rows enlarged 6 times. 
KEY TO SUBGENERA. 
Upper third molar with three closed triangles, 
lower second molar with four closed sections 
and usually a posterior loop; lower third molar 
generally with three closed triangles. ....:...... 
Upper third molar with two closed triangles; 
lower second molar with anterior pair of angles 
confluent; lower third molar with three trans- 
Verse lOOPS Ae 2 ges.ces -eeSaeeene a Renan 
Upper third molar with two closed triangles; 
lower third molar with two closed triangles and 
two: transverse loops: ace..ea dea) ae eee 
Upper third molar with three closed triangles 
and one crescentic loop; lower third molar 
with two closed triangles, and two internal 
A. Microtus. 
ET gerard \ Ticrotus 
1s a UES 
. .Orthriomys 
..Herpetomys 
PAGE 
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; 
