8G 



molars in functional ])ositiou behind, 1 examined the jaw beneath, Init fouiKl ' 

 110 trace of a successor. 



The portion of jaw is of more uniform depth, and the base less convex 

 than in Hyopsodus. It is also more impressed and concave below the position 

 of the back molars. The space occupied l\y the molars is about equal to 

 that in the smaller specimens of Hyopsodus. 



Perhaps the sjiecimen may pertain to Microsus, or prol^ably may l)elong to 

 a genus difTerent from either of those just named. Its measurements are as 

 follows : 



Lines. 



Space occupied by tlie last premolar aud molars 7f 



Space occupied by tbe molars ■ . . 5| 



Depth of jaw at fore part of last molar '3^ 



Depth of jaw at last premolar Sf 



Anteroposterior diameter of last molar 2i 



Anteroposterior diameter of last i^remolar If 



NOTHARCTUS. 



NOTHAKCTUS TENEBROSUS. 



A small extinct pachyderm, referred to a genus with the above name, judg- 

 ing from the anatomical characters of the specimen upon which it was founded, 

 was probably as carnivorous in habit as the raccoon and bear. The specimen 

 to which I allude, represented in Fig. 36, Plate VI, consists of tbe right 

 ramus of a lower jaw with most of tbe teeth. It was discovered during Pro- 

 fessor Hayden's exploration of 1870, on Black's Fork of Green River. I at 

 first viewed it as pertaining to a carnivorous animal, and thus referred it ; but 

 tbe anatomical relations of the specimen with those of remains of other ani- 

 mals which have been found in association with it have led me to view the 

 jaw as having belonged to a pachyderm. The ramus of the ja^v contained a 

 series of seven molar teeth, all of which are preserved except the first pre- 

 molar'. A well-developed canine occupies a position immediately in advance 

 of the molar series, and the incisors filled the interval between the canines of 

 the two sides. Thus the teeth of the lower" jaw of Notliarctus form an un- 

 broken arch. The incisors are lost from the specimen, and the condition of 

 the alveoli is such that the number of them cannot be ascertained. 



The canine tooth of Notharctus in its relative position, form, and propor- 

 tions resembles that of ordinary carnivores. It curves from the opening of 

 the alveolus slightly backward witli an inclination outward. The crown is 



