South Dakota School of Mines 8 7 



The earliest specimens of the rodents obtained were found 

 by Harden in the Big Badlands, and described by Leidy. With 

 the exception of two other species described many years ago by 

 Cope, little further information became available until the last 

 few years, during which time Mr. Peterson of the Carnegie 

 Museum, and Mr. Matthew of the American Museum of Natural 

 History, each described a number of species.* The Carnegie 

 Museum material has come chiefly from northwestern Nebraska 

 and eastern Wyoming, the American Museum material from 

 Little White river. 



The commonest fossil is Steneofiber. This is especially 

 abundant in the Lower Rosebud beds of Little White river 

 and in the Harrison (Daemonelix) beds in northwestern Ne- 

 braska and in eastern Wyoming. Entoptychus, the gopher-like 

 rodent, seems to be fairly common in the Little White river 

 area also. Peterson found many specimens of Steneofiber fossor 

 in close association with the Devil's Corkscrews of the Harrison 

 beds and. as referred to elsewhere, suggests the reason for the 

 association. This animal was smaller generally than the present 

 day beaver. Its skull is comparatively large, the lower jaws heavy, 

 neck short, limbs and feet powerful, tail round, rather heavy 

 and of moderate length. Peterson states that the limb presents 

 a striking similarity to that of other burrowing rodents and 

 approaches that of the mole in its position. The elongated and 

 narrow scapula of the mole, the heavy clavicle, the strongly built 

 humerus, and the broad foot with the long and powerful un- 

 guals, is rather suggestive of the habits of this animal, which 

 was probably burrowing to a considerable degree. The animal 

 is related to the beaver, but is evidently not in the direct line of 

 ancestry. 



The following is a list of all species determined up to the 

 present time : 



Middle Oligocene. 



Castoridae — Ancestral beavers. 



Eiitypomys thomsoni. Matthew. 

 Ischyromidae — Ancestral squirrels and mormots. 



Ischyrotnis typus, Leidy. 



*Peter>iK>n, O. A. Description of New Rodents and Discussion of 

 the Origin of Daemonelix. Mem. Carnegie Mus.. Vol. 2, 1905, pp. 139- 

 202. 



Matthew. W. D A Lower Miocene Fauna from South Dakota. 

 Bull. Am. Mus. Nat. Hist., Vol. 23, 1907. pp. 169-219. 



