298 VERTEBRATE ANIMALS OF THE UNITED STATES 



C. cristata (Erxleben). Hooded seal. Color bluish black, with 

 whitish spots; length 2,800 mm.: North Atlantic; southward occasion- 

 ally to Long Island. 



Order 8. Rodentia. — Rodents. Small or medium-sized mammals 

 with long chisel-like incisor teeth, no canines, and grinders (premolars 

 and molars) which are separated from the incisors by a wide space, the 

 incisors having persistent papillae and growing continuously (Fig. 

 168); feet mostly pentadactylous and plantigrade; placenta discoidal: 

 about 1,400 species grouped in about 21 families, constituting a third 

 of all mammals and occurring on all the continents; about 400 species 

 in the United States. 



Key to the United States Families of Rodentia 



El 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. Muridce. 



h2 Hind legs greatly elongated; jumping 



mice 4. Zapodidce. 



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

 it At least 4 well developed grinders in each jaw. 



gi Tail very short; mountain beavers 6. Aplodontiidce. 



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



d2 Long quills present in the fur; porcupines 5. ErethizontidcE. 



C2 Tail large and flattened; beavers 8. CasioridcB. 



di Four upper incisors present; rabbits and hares 9. LeporidcB. 



a2 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, 2,l3)'- over 700 species and 100 

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

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

 grouped in 4 subfamilies. 



