456 University of California Publications in Zoology [Vol. 12 



frondator, leucodonta, and belugae, at least on the evidence here sub- 

 mitted, though canadensis would seem to be separated off by its 

 narrow tail. 



General cranial characters 



Crania compared. — Castor canadensis frondator, nos. 60354 $ , 

 35946 9 , U. S. Nat. Mus., from the Colorado River, Mexico, 15 miles 

 south of Yuma, Arizona, and from the San Pedro River, Mexico, 

 respectively; C. c. pacificus, nos. 3672, 71276 2 , 71812 $ , 71814 9 , 

 87628 2 , 87629 S , 126190 $ , U. S. Nat. Mus., all from the mainland 

 of Washington State ; C. c. canadensis, nos. 174525 £ and 174526 9 , 

 U. S. Nat. Mus., Nepisiquit River, New Brunswick ; C. c. belugae, nos. 

 4347 $ , 4225 3 , 4224 $ , Mus. Vert. Zool., Cook Inlet region, Alaska. 



Nasals of different outline in frondator than in pacificus. Their 

 lateral outlines converge more regularly in a posterior direction, there 

 being no tendency to be parallel, as is the case in comparable skulls 

 of pacificus. The ratio of width of nasals to length tends to be 

 greater in frondator than in pacificus, though there is decided over- 

 lapping. Measurements of foramen magnum similar, though most 

 specimens of pacificus have their foramina broader in proportion to 

 vertical diameter than is the case in frondator. Teeth in one specimen 

 of frondator flare more laterally than in any specimen of pacificus 

 (see fig. I, p. 442, frondator, no. 60354, 34.3 mm., 35946, 30.9; 

 pacificus, no. 3672, 33.1 mm., 71276, 31.3, 71812, 30.4, 71814, 34.1, 

 78395, 32.5, 87628, 32.6, 87629, 34.3). Ratio of maxillary tooth-row 

 to basilar length greater in frondator. Tooth-row longer in frondator 

 no. 60354 than in pacificus no. 71812. These specimens have basilar 

 length identical. Tooth-row longer in frondator no. 35946, with 

 basilar length of 111.7 mm., than in pacificus no. 71812, with basilar 

 length of 118.7. 



Median process in interpterygoid fossa longer and sharper in 

 frondator than in pacificus. 



These differences between frondator and pacificus are for the most 

 part slight. The outline of nasals is the most dependable single sep- 

 arative character. 



Measurements fail to separate frondator either from canadensis or 

 the Cook Inlet race. Distance from the inion to dorsal outline of 

 foramen magnum less in frondator than in canadensis from New 

 Brunswick (frondator, no. 60354, 18.9 mm., 35946, 18.0; canadensis, 

 no. 174525, 20.9 mm., 174526, 19.9). Ratio of maxillary tooth-row 

 to basilar length slightly greater in frondator than in the New Bruns- 



