S() l 2 BULLETIN 56, UNITED STATES NATIONAL MUSK I M. 



Measurements of 13 specimens of Castor canadensis frondator.a 



a All but the last two specimens are in the collection of the American Museum of Natural History. 

 b Typo. 



Family MURIDyE. 



RATS AND MICE. 



Dentition.— I. g; C. §E§; P. S M - fcl= 1( >- 



Skull with contracted frontals; a short and slender jugal, generally 

 reduced to a splint between the zygomatic processes of the maxilla 

 and squamosal; the lower root of the former process more or less 

 flattened into a perpendicular plate: typically, the infraobital vacuity 

 tall, and wide above and narrow below. Lower incisors compressed; 

 no premolars; molars rooted, or rootless, tuberculate, or with angu- 

 lar enamel-folds. Pollex rudimental; tail generally nearly miked and 

 scaly. ( Flower and Lydekker.) 



