342 



ON THE EXTINCT MAMMALIA OF 



be in error in supposing such a change possible, and have not had the opportunity of 

 examining skulls of equally aged individuals of the existing Beaver, to ascertain 

 whether any change of the kind really occurs. In their present condition, the width 

 of the crown greatly exceeds the antero-posterior diameter, especially in the second 

 molar, and the lines of enamel on the triturating surfaces proceed in oblique curves 

 from without inward and backward. What is the internal valley, as ordinarily seen 

 in the corresponding teeth of the recent Beaver, in the fossil opens at the back of the 

 crown. 



Comparative measurements of the fossil with the corresponding portion of the skull 

 of a recent Beaver, are as follow : 



Length of space occupied by four molars, 

 Length of hiatus from first molar to incisors, 

 Breadth of fiice opposite first molars, 

 Breadth of face in advance of infra-orbital foramina, 

 Width of palate between first molars, 

 Width of palate between last molar alveoli, 

 Antero-posterior diameter of first molar. 

 Transverse " " 



Antero-posterior diameter of second molar, 

 Transverse " " 



Antero-posterior diameter of third molar, 

 Transverse " " 



Diameter of incisors, 



MURWM 



This family is represented in the miocene fauna of the Mauvaises Terres by a genus 

 to which I have given the following name : 



EUMYS. 



EUMTS ELEGANS. 



A specimen discovered by Di-. Hayden at the head of Bear Creek indicates a rodent 

 of the Rat family. It was found in what Dr. Hayden has named "the Turtle and 

 Oreodon bed," or bed B of his vertical section of the Dakota and Nebraska tertiary 

 deposits. The specimen, represented in figure 12, plate XXVI, twice the natural 

 size, consists of a portion of the left side of a lower jaw, and belonged to an animal 



