DAKOTA AND NEBRASKA. 201 



The temporary molar is inserted by a pair of widely separated fangs, and its crown 

 presents the usual greater breadth than the succeeding pair of those of the teeth 

 behind, as in pachyderms generally. The crown is trilobate externally and 

 internally, and this condition probably corresjoonds with three constituent pairs of 

 lobes, the distinction of which is for the most part obliterated by wearing^ The 

 median division of the crown is largest, and that in advance is the smallest. The 

 abraded summit of the former presents an irregularly transverse quadrate surface of 

 exposed dentine, continuous with a smaller subreniform tract upon the anterior 

 division. The posterior division of the crown still exhibits the distinction of a trans- 

 verse pair of lobes, of Avhich the outer one is much the larger. This joresents on its 

 abraded summit a crescentoid surface of exposed dentine, and the inner one a minute 

 circular islet of the same substance, and both are considerably below the level of the 

 worn surfaces of the divisions of the crown in front. 



The two permanent true molars, preserved in the specimen, are alike in form and 

 size, and the anterior portion of the last molar agrees in character with the corres- 

 ponding portion of the teeth in advance. 



The crown of the first and second true molars is composed of two transverse pair 

 of conical lobes, of which the anterior are about a third higher than the posterior, and 

 are separated from them by a deep transverse valley. The inner and outer lobes are 

 separated by a valley about half the depth of the former, closed at the fore and back 

 part of the crown by a small tubercle. The front tubercle is most conspicuous, and 

 receives a feeble offset or ridge from the antero-external lobe. A similar offset from 

 the postero-external lobe ends in the middle of the transverse valley of the crown. 

 A basal ridge exists nowhere except at the fore part of the crown, where it is most 

 conspicuous externally, and is there associated with the tubercle closing the front of 

 the fore and aft valley of the tooth. A minute circular islet of exposed dentine 

 occupies the summits of the anterior lobes of the crown of the first true molar. 



The breadth of the crown of the last temporary molar is 2i lines. The breadth of 

 the crown of the second unworn permanent true molar is 1| lines, and its height at 

 the anterior division is the same. 



The depth of the jaw fragment below the first permanent true molar is one-fourth 

 of an inch. The base is moderately convex fore and aft. 



Though I have found it difficult to ascertain, by comparison with figures, how far 



the fossil described differs from the corresponding portion of other known animals, it 



nevertheless appears to me to do so sufficiently to refer it to a distinct genus and 



species. 



26 



