86 



molars in functional position behind, I examined the jaw beneath, but found 

 no 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 Ihe position 

 of the back molars. The space occupied by the molars is about equal to 

 that in the smaller specimens of Hyopsodus. 



Perhaps the specimen may pertain to Microsus, or probably may belong to 

 a genus different from cither of those just named. Its measurements are as 

 follows : 



Lines. 



Space occupied by the last premolar and molars 7f 



Space occupied by the molars . . 5| 



Depth of jaw at fore part of last molar 3£ 



Depth of jaw at last premolar 3£ 



Anteroposterior diameter of last molar 2\ 



Antero-posterior diameter of last premolar 1| 



NOTHARCTUS. 



NOTHARCTUS TENEBROSUSu 



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 the right 

 ramus of a lower jaw with most of the 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 

 the 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 jaw 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 Notharctus 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 with an inclination outward. The crown is 



