Order RODENTIA 



129 



10. Skull length more than 90 mm. (3\4 in.) ; length of incisive 



foramen, fig. 73d, shorter than length of grinding sur- 

 faces of first two upper cheek teeth beaver, Castor canadensis 

 Skull length less than 75 mm. (3 in.) ; length of incisive 

 foramen greater than length of grinding surfaces of first 

 2 upper cheek teeth 11 



11. Upper jaw with 4 cheek teeth on each side 



meadow jumping mouse, Zapus hudsonius 



Upper jaw with 3 cheek teeth on each side 12 



12. Skull length more than 45 mm. (1^4 in-) 



muskrat, Ondatra zibethicus 



Skull length less than 45 mm 13 



13. Skull length 30 mm. ( 1 1 s in.) or less 14 



Skull length more than 30 mm 23 



Fig. 74. — Additional characters used in the identification of ro- 

 dents: grinding surface of molars of a, rice rat, b, eastern u I 



rat, c, Norway rat, d, meadow vole, e, prairie vole; side view oi 

 rostrum of, /, house mouse, g, golden mouse, //, white-footed mouse ; 

 i, front of rostrum of southern bog lemming; j, part of skull of 

 southern flying squirrel, top view; k, skull of fox squirrel, top view. 



