454 University of California Publications in Zoology l v ° L - 12 



tensis, and in the following additional instances. Nos. 12668 and 

 16384, the youngest examples of subauratus, have foramina magna of 

 larger dimensions than the older crania of that subspecies, and are not 

 so characteristically marked off from other species of beavers. Fur- 

 thermore, no. 4232, from the Cook Inlet region, has as short a vertical 

 diameter of foramen magnum as has subauratus, as have also two 

 very young specimens of leucodonta (nos. 12105, 12106), and one 

 specimen of pacificus (no. 71276). 



This measurement, the vertical diameter of the foramen magnum, 

 is less, with the above exceptions, than in any other specimen of beaver 

 measured. The transverse diameter averages less than in phaeus, 

 although it averages about the same as in leucodonta, pacificus and 

 belugae, and greater than in frondator or canadensis from eastern 

 Canada. 



The ratio of the vertical diameter of the foramen magnum to the 

 basilar length brings out concretely its different shape. This ratio 

 is nearly forty-six percent (taken on the basis of comparison of aver- 

 ages derived from table X, opposite p. 438) greater in leucodonta and 

 fifty-one per cent greater in frondator than it is in subauratus. It 

 will be noted that the ratio decreases with age, the foramen magnum 

 in the youngest example (no. 16384) being nearest to the canadensis 

 series (see table XX, p. 449). 



(3) The ratio of the width of nasals to basilar length is greater 

 in every specimen of subauratus, and in four of the five specimens 

 of shastensis, than in any other subspecies of western beaver measured 

 (except several specimens of belugae, and three examples of leuco- 

 donta). 



(4) The ratio of the length of maxillary tooth-row to basilar 

 length averages greater in comparable skulls of subauratus than in 

 pacificus, its nearest neighbor on the north, but practically the same as 

 in frondator, its nearest neighbor on the south. 



CASTOR CANADENSIS FRONDATOR MEARNS, FROM THE COLORADO 

 AND SAN PEDRO RIVERS, MEXICO 



Material 

 Three specimens from United States National Museum: San 

 Pedro Eiver, Mexico, 1 (no. I'^f?, skin and skull) ; Lagoon of Colo- 

 rado River, about 15 miles south of Yuma, Arizona, 2 (nos. 60354, 

 60355, skulls only). 



