290 \'ERTEBRATE ANIMALS OF THE UNITED STATES 



Key to the United States Families of Rodentia 



ai Tail present. 



bi Two upper incisors present. 



Ci Tail cylindrical or compressed laterally (except flying-squirrels). 

 di No quills in the fur. 



ei Body stout; fore feet with digging claws; pocket 



gophers 2. Geomyidce. 



e2 Fore feet not with digging claws. 



fi Not more than 3 well-developed grinders in 

 each jaw. 

 gi No cheek-pouches present. 



hi Hind legs not greatly elongated; rats and 



mice I. Murida. 



h-i Hind legs greatly' elongated; jumping 



mice 4. Zapodida;. 



g2 Large cheek-pouches present; pocket mice. 3. Heteromyidce. 

 {0 At least 4 well developed grinders in each jaw. 



gi Tail very short; mountain beavers 6. Apoldontiida. 



g2 Tail long or moderate; squirrels; marmots. . 7. Sciuridcc. 



di Long quills present in the fur; porcupines 5. Erethizontidm. 



C2 Tail large and flattened; beavers 8. Castor idee. 



hi Four upper incisors present; rabbits and hares 9. Leporidcc. 



0.1 External tail wanting; pikas 10. Ochotonidce. 



Family I. Muridae. — Rats and mice. Small, slender rodents with 

 a long tail, with feet not specially modified and with naked soles; snout 

 elongated; nostrils separated by a furrow; eyes and ears large; upper lip 

 usually divided; dentition i/i, 0/0, 0/0, 3/3: over 700 species and 100 

 genera, comprising half of all rodents and being the largest family of 

 mammals; distribution cosmopolitan; 150 species in the United States, 

 grouped in 4 subfamilies. 



Key to the SubfamiUes of Muridae 



ai Molars of upper jaw with a more or less tuberculate biting surface 

 (Fig. 160). 



bi Tubercles arranged in 3 longitudinal rows i. Murince. 



hi Tubercles either arranged in 2 rows (Fig. 160) or flattened. ... 2. Cricetina:. 

 a2 Molars with a flattened surface and not tuberculate (Fig. 167). 



bi Tail long; body slender 3. Neotomince. 



hi Tail short; body stout 4. MicrolincB. 



Subfamily I. Murinae. — Old world rats and mice. Form slender; 

 molars of upper jaw with 3 longitudinal rows of tubercles on their 

 crowns; tail long and scaly or scantily haired; thumb rudimentary: 

 130 species, in all parts of the old world except Madagasc^-r; a few 

 species have followed man to the new world. 



