MEMOLTRS 
OF THE 
CARNEGIE MUSEUM. 
WOE ar NO. 4. 
DESCRIPTION OF NEW RODENTS AND DISCUSSION OF THE 
ORIGIN OF DASMONELIX. 
By O. A. PErerson. 
One of the field parties of the Carnegie Museum with the writer in charge was 
detailed by the Director to collect fossils in western Nebraska and eastern Wyo- 
ming during the season of 1904. In the collection which was secured are some very 
complete remains of rodents, belonging to two new species of Steneofiber not pre- 
viously described. This material fully substantiates the views foreshadowed by 
Professor Scott, Dr. Matthew, and others, that this genus is clearly distinct from 
Castor. In the following detailed description of this new material it will be seen 
that there are some features, especially in the cranium, which are similar to those 
in the genus Castor, but these do not necessarily imply relationship. The skeleton 
as a whole shows osteological characters very similar to those of Aplodontia rufa 
with which I have compared it. Cynomys ludovicianus is also used for comparison 
in studying the present fossil forms. 
The material was discovered in the Upper Miocene sandstones, or Harrison 
(Demonelix) beds, in Sioux Co., northwestern Nebraska, and also in Converse Co., 
Wyoming, immediately across the Nebraska-Wyoming state line, and in the same 
general locality. Fossils are comparatively rare in this horizon. The list of genera 
found is as follows: Promerycochwrus, ? Mesoreodon, a Peccary, Oxydactylus, and 
other small species of camels not identified. The commonest fossils from this 
horizon are the new species of rodents described in the following pages. 
I take pleasure in acknowledging the valuable assistance of Mr. Earl Douglass 
1A specimen of Aplodontia rufa (Col. U. S. Dept. Agri., No. 77975) was kindly furnished for comparison by Dr. C. 
H. Merriam, Smithsonian Institution, Washington, D. C. 
