838 ON THE EXTINCT MAMMALIA OP 



impression of the inasseter is comparatively feeble, and only reaches as far forward as 

 the position of back part of tlie second molar tooth. 



Of the inferior molar teeth, two complete series, viewed on their triturating surface, 

 and magnified three diameters, are represented in figures 5 and 6. The former figure 

 represents those of the left side considerably worn ; the latter represents those of the 

 right side still more worn. 



Tiiese teeth are four in number, and more uniform in appearance among themselves 

 than those above, of which they resemble the third and fourth in a reversed condition. 

 The anterior transverse ridge of the triturating surface in the two upper teeth just 

 indicated, in the lower molars occupies a position at the back of the crown. When 

 much worn, the teeth viewed on their triturating surfaces, as seen in figure 6, appear 

 to have the crown trilobed internally and bilobed externally. 



Measurements derived from several of the specimens are as follow : 



Lines. Lines. Lines. Lines. 



Length of the lower molar series, . . .7 7 7i 7 



Depth of lower jaw below second molar, . . 5J 5 



Depth of lower jaw in advance of the molars, . 5 4 J 



CASTOEIDM. 



The family of the Beaver is represented both in the miocene fauna of the Mau- 

 vaises Terres and the pliocene fauna of the Niobrara River. 



PAL.:EOCASTOR. 



Pal^ocastor nebrascensis. 



A rodent of the Beaver family, and about half the size of the existing Beaver, is 

 indicated by several specimens contained in Dr. Hayden's collection of fossils from 

 the Mauvaises Terres of White River. 



One of the specimens, represented in figures 7, 8, plate XXVI, consists of a much 

 mutilated skull, with the fragments retained nearly in their original position by a 

 mass of included calcareous matrix. The greater part of the cranium is destroyed, 

 as is also the nose and parts of the jaws, though all the teeth are preserved. A 

 second specimen consists of portions of both jaws of an aged individual, badly muti- 

 lated and imbedded in a mass of matrix. The remaining specimens, from a third 

 individual, consist of portions of both sides of the lower jaw and one side of the upper 

 jaw, containing all the molar teeth, and freed from investing matrix. 



So fiir as any conception can be had of the form and construction of the skull from 

 the mutilated specimen above mentioned, it appears to have resembled that of the 

 closely allied animal Steneofiber viciacensis, an extinct rodent from the lower pliocene 



