1916] Taylor: Beavers of Western North America 435 



urement in shastensis no. 50977 exceeds that in subauratus no. 

 12668, which has the same basilar length, by 8.9 percent. The 

 difference holds throughout the scries in specimens of comparable 

 age. 



The fronto-maxillary suture, situated dorsally on skull between 

 backward-extending tongue of premaxilla and malar, is longer in 

 shastensis than in subauratus. This holds for all specimens, regard- 

 less of age (see tables of measurements, pp. 436 and 449.) 



The interparietal is somewhat broader in all specimens of shas- 

 tensis, old and young, than it is in any specimen of subauratiis. 



Remarks. — The new subspecies, while clearly marked off from 

 the beaver of the San Joaquin Valley by a number of valid cranial 

 characters, nevertheless finds in the golden beaver its closest ally. 

 This is shown by the facts that: (1) It is nearly identical with 

 Castor subauratus subauratiis in many cranial dimensions; (2) its 

 foramen magnum shows the same general proportions; (3) its 

 process medially in the interpterygoid fossa is nearly identical 

 with that in subauratiis, being different in form from that of any 

 other west American beaver. 



Cassel, Shasta County, California, the type locality of the new 

 form, is situated on Hat Creek, a tributary of the Pit River, which 

 is in turn a tributary of the Sacramento River. The two forms, 

 Castor subauratiis subauratiis and Castor subauratus shastensis, are 

 found in the same hydrographic basin, namely that draining into 

 San Francisco Bay. It should be noted, however, that the type 

 locality of the Shasta beaver is on the eastern slope of the main 

 chain of the Sierra Nevada Mountains. The surrounding region is 

 characterized by environmental conditions probably much more 

 typical of the Great Basin faunal area than of those of the Sacra- 

 mento Valley. It is entirely possible, if not probable, that the Pit 

 River Narrows at present constitute a barrier not regularly crossed 

 by beavers. The limits of the range of Castor subauratus shastensis 

 are yet to be defined. There would seem to be a possibility that 

 the beaver of the Great Basin will be found to be referable to it. 



Material. — Five specimens, skulls only, all loaned to the writer by 

 the authorities in charge of the Biological Survey mammal collection, 

 United States National Museum : Cassel, Hat Creek, Pit River, Shasta 

 County, California (nos. 50976-50979, 51477). 



