New Species of Aplodontia. 325 



collection, and the average size of this foramen is larger in the 

 California than the Washington animal. 



The second and third points relate to the audita] bullae and 

 pterygoid fossa, which parts Peters states are entirely like those 

 of Arctomys. The resemblances in these parts in the two genera 

 are hard to find, while the points of difference are numerous and 

 striking. 



It is difficult to understand how a naturalist of Peters's ex\ie- 

 rience could be led to state that much similarity exists between 

 the large, roundish, and somewhat inflated audital bullae of Arc- 

 tomys, and the small, irregularly flattened, and transversely 

 elongated ones of Aplodontia. The pterygoid fossae are even 

 more dissimilar. In Arctomys this fossa is more than twice as 

 long as in Aplodontia, it is constricted in the middle, and its 

 walls terminate posteriorly in long hamular processes. More- 

 over, its anterior boundary bears a long, slender spine which pro- 

 jects backward from the palate. In Aplodontia the pterygoid 

 fossa is remarkably short, its sides are parallel, and its anterior 

 boundary is formed by a notch into the palate. Indeed, should 

 the above mentioned resemblances to Arctomys really exist, it 

 will be necessary to constitute a new genus for the reception of 

 Peters's specimen. 



Turning now from cranial to external chsracters, but two 

 points remain for consideration. The shortness of the tail spoken 

 of (" 6 lines") is perfectly normal. Richardson, in his measure- 

 ments " of a full-grown specimen," gives exactly the same length 

 (6 lines).* Peters used Baird's measurement of 10 lines as his 

 standard of comparison, but Baird expressly stated that his 

 measurements were from a mounted specimen (North American 

 Mammals, 1847, p. 35*3). However, the tail presents considera- 

 ble individual variation, sometimes, according to Suckley, at- 

 taining the length of 1^ inches; and the material now in hand 

 does not indicate that the tail of A. rufa is longer than that of 

 A. major. 



The white spot on the flanks, which is stated to be 15-20 mm. 

 in diameter, roundish, and symmetrical on the two sides, is a 

 peculiarity which may prove to be characteristic of a very distinct 



* Fauna Boreali- Americana, 1829, p. 212. 



