DAKOTA AND NEBRASKA. 225 



No superior incisors of R. ocddentalis are preserved with any of the specimens of 

 jaws we have had an opportunity of examining. An isolated upper lateral incisor 

 is supposed to belong to this species, as it holds about the same relation of size and 

 form to the opposed tooth of the lower jaw in Prof. Hall's collection, that the corres- 

 ponding tooth in the Indian Rhinoceros does to the lower lateral incisor. It has a 

 compressed cylindroid fang nearly an inch and a half long, with an oblong crown ten 

 and a half lines wide and five lines thick. Internally the crown is worn away, in a 

 shelving, somewhat concave manner, to the base posteriorly. 



Several fragments of jaws with molar teeth apparently indicate considerable range 

 in the size of individuals of R. occidental is. It is not improbable that the largest 

 specimens may pertain to another species, but, so far as characters have been pre- 

 served, a variation in size is the only important difference observed. 



Figure 1, plate XXIII, represents an imperfect tooth, a sixth upper molar, 

 considerably larger than the corresponding tooth in the skull of Dr. Owen's 

 collection. It measures twenty-one lines fore and aft, and twenty-two and a half 

 lines transversely, while the other measures nineteen lines fore and aft and twenty 

 lines transversely, 



A much mutilated jaw in Dr. Evan's collection has the series of six molars nearly 

 eight inches in length, while the same series in the specimen of Prof. Hall measures a 

 little over six inches. Another fragmentary specimen exhibits the same series about 

 six and a half inches in length. 



Figures 2, 3, plate XXIII, represent the crowns of two lower lateral incisors, having 

 the same form as the corresponding teeth in the lower jaw of Prof. Hall's collection, 

 but much larger. These specimens measure at base ten and a half lines wide and six 

 lines thick. 



Another isolated specimen of the crown of a lower lateral incisor is a little larger 

 than those in the jaw just indicated, as it measures eight and a quarter lines wide 

 and five and a quarter thick. 



The collection of Prof Hall contains specimens of the posterior two upper molars of 

 the teraporaiy series, and fragments of the lower jaw with the last temporary molar 

 and the first permanent true molar. The upper temporary molars differ from their 

 permanent successors in having the inner lobes more equally developed and separated 

 internally to their base, in having several accessory folds projecting into the bottom 

 of the principal valley, and in not having the basal ridge continuous internally nor 

 strong. 



Whether Rhinoceros occiclentalis possessed a horn cannot with certainty be ascer- 

 tained from the specimens we have had the opportunity of examining. There is no 

 process, prominence, or roughness upon the forehead indicating the existence of a 

 frontal horn, and the comparatively depressed condition of the face approaching the 



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