8 Journal of the Mitcheix Society. [March 



aa. Grinding- teeth with tubercles in two rows or without dis- 

 tinct tubercles of any kind, 

 b. Crowns of grinding- teeth with tubercles in two rows, 

 bb. Crowns of grinding- teeth divided into loops or triangles, 

 (subfam. Cricetinae, American Rats and Mice), 

 c. Upper front teeth grooved, size small. 



Genus Reithrodontomys. 

 cc. Upper front teeth not grooved, 

 d. Tail as long as or longer than head and body, total 

 length of adults 9 inches or over, fur coarse. 



Genus Oryzomys. 



dd. Tail much shorter than head and body, total length 



of adults 9y2 inches or over, form stout, fur 



coarse. Genus Sigmodon. 



ddd. Total length of adults under 8 inches, fur fine and 



soft, belly wiiite. Tail as long as head and 



body or somewhat shorter. Genus Pekomyscus. 



e. Upper front teeth narrow, compressed; tail always 



eyes and ears large, belly white, (subfam. 



Neotominae, Wood Rats and Cave Rats). 



Genus Neotoma. 

 ee. Upper front teeth bi'oad; tail usually short; ears and 

 eyes small; belly g-enerally not white (sub- 

 fam. Microtinae, Voles and Lemmings), 

 f . Lower front teeth short, their roots terminating on 

 inner side of grinders (upper front teeth 

 grooved in our species). 



Genus Synaptomys. 

 ff. Lower front teeth long, their roots extending under 

 posterior grinding teeth into outer side 

 of jaw. 

 g. Tail flattened sideways, size large. Genus Fiber. 

 gg. Tail round, size small or medium, 

 h. Grinding teeth without roots (prongs). 



Genus Microtus. 

 hh. Grinding teeth with roots (prongs) in adults. 



Genus Evotomys. 



