422 



University of California Publications. 



[Geology 



Virgin Valley and Thousand Creek, Humboldt County, Nevada. 

 The collections made by the expedition have been presented to 

 the University of California by Miss Alexander. 



The writer is greatly indebted to Professor John C. Merriam 

 for the privilege of working over the material, and for his 

 courtesy and assistance in the work. 



With the exception of isolated specimens, the rodent remains 

 were found only in a few localities. In these places the bones 

 were scattered abundantly over small areas, and the collectors 

 by dint of hard labor on their hands and knees, sifted over every 

 inch of the ground in order that the smallest bones and teeth 

 should not be overlooked. 



As rodents are generally restricted in their geographic range, 

 and are relatively good indicators of climate and other physical 

 conditions, they are an important factor in the work of building 

 up a picture of the conditions which obtained in any region in 

 past time ; and it is hoped that the following list of forms will 

 help materially in determining the nature of the environment in 

 which mammalian life existed in the Virgin Valley and Thousand 

 Creek region during the late Tertiary. 



ARCTOMYS NEVADENSIS, n. sp. 

 Figs, la, lb and 2. 

 Type specimen no. 12506, Univ. Calif. Col. Vert. Palae., an- 

 terior portion of a left ramus of the lower jaw with P 4 and M 1 

 intact, and the incisor broken off where it emerges from the jaw. 

 From locality no. 1105, near Thousand Creek, Humboldt County, 

 Nevada. 



Specific Characters. — Incisors small in proportion to the other 

 teeth. P 4 and M x relatively large, more nearly square in cross- 

 section, and set more obliquely in the jaw than in other species. 

 A small ridge on the anterior face of P 4 instead of a tubercle. 

 The symphysis ends posteriorly in front of the mental foramen 

 instead of extending back of it as in other species. 



General Description. — P 4 and M x may be said to consist of 

 two distinct portions, an anterior formed by a large protoconid 

 and metaeonid with parastylid between, and a posterior consist- 



