Order RODENTIA 



129 



10. 



11. 



12. 



13. 



Skull length more than 90 mm. (31/ in.) ; length of incisive 

 foramen, fig. 73(i, 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 



Upper jaw with 4 cheek teeth on each side 



meadow jumping mouse, Zapus hudsonius 



Upper jaw with 3 cheek teeth on each side 12 



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



muskrat. Ondatra zibethicus 



Skull length less than 45 mm 13 



Skull length 30 mm. (11/^ 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 wood 

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

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

 /, front of rostrum of southern bog lemming; ;, part of skull of 

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



