DAKOTA AND NEBRASKA. 223 



Lower jaw. — The specimen of the lower jaw oi Rhinoceros occidentalis, in Professor 

 Hall's collection, has lost the portion back of the teeth. It contains the full molar 

 series and all the incisors. 



The body of the jaw containing the molars is of more uniform depth than in the 

 Indian Rhinoceros, and the base is less convex fore and aft. The chin does not pre- 

 sent the broad sloping plane of the species just mentioned, but is comparatively 

 narrow and convex, and recalls to mind the condition of that of the Tapir. The 

 symphysis measures thirty-five lines in length. 



Dentition. — The formula of dentition of R. occidentalis appears to be the same as in 

 the existing unicorn Rhinoceroses, — that is to say, two incisors and seven molars, on 

 each side of the face, in both jaws. 



The upper molar teeth generally of the genus Rhinoceros have cuboidal crowns 

 composed of an outer pair of connate lobes, with a pair of pyramidal lobes or folds 

 extending from them inwardly. The bottom of the crown is furnished with a basal 

 ridge, variable in its degree of development. A transverse valley separates the inner 

 lobes, and expands inwardly. A valley of secondary importance is constituted by the 

 interval between the postero-iuternal lobe and the back portion of the basal ridge. 

 Accessory folds or prominences, in diflferent species, extend from the principal lobes 

 into the principal valley. The first molar is much smaller than the others, and is 

 rudimental in its form. The succeeding three molars present sufiicient distinction of 

 structure from those behind to be recognized as premolars. In the last true molar 

 the postero-internal lobe is usually obsolete. In the unworn teeth the constituent 

 lobes of the crown present more or less continuous acutely enamelled summits. 

 These are rapidly blunted by attrition in the process of mastication, and tracts of 

 dentine become exposed and gradually widen as trituration proceeds. From the 

 union or confluence of parts or processes of the lobes towai'ds their base, when their 

 summits are worn away to the level of the points of confluence, portions of the valleys 

 become pits, and appear as such upon the broad exposed dentinal tracts of the mas- 

 ticating surfaces. 



The upper molar teeth of -B. occidentalis, plate XXII, figures 1, 2, bear a close re- 

 semblance to a series represented in De Blainville's Osteographie, G. Rhinoceros, plate 

 xii, as belonging to R. incisivus, from Auvergne, France. They, however, do not 

 bear the same resemblance to those of Aceratherium incisivum, which De Blainville 

 considered the same as the former, as represented in tab. xiv of Kaup's Ossemens 

 Fossiles. 



The upper true molars are provided with a thin basal ridge, which is obsolete on 

 the inner side of the internal lobes and is best developed in front and back of these 

 and between them. The principal valley deepens inwardly, but is shallowest just 



