RHINOCEROS OF NEBRASKA. 



555 



posterior valley. Its principal valley is intruded upon only by a slight bulge at 

 the middle of the antero-internal lobe. 



In the corresponding lobe to the latter in the preceding two molars, the bulging 

 posteriorly successively increases, and also exists anteriorly. This dilatation of the 

 antero-internal lobes decreases the depth externally of the anterior valleys, so that 

 they slope downwards from their entrance, and in the trituration of the teeth they 

 are obliterated from without inwards, leaving no islets of enamel behind as in the 

 premolars. 



In the specimen under consideration, the abrasion of the first true molar has 

 effaced the outer half of the anterior valley. 



In the second to the fourth premolars inclusive, the inner lobes are confluent 

 internally at their bases, but to a less extent in the anterior than the remaining 

 two of those designated. From trituration, the latter in the specimen exhibit the 

 remains of the anterior valleys as simple trilateral pits of enamel occupying the 

 centre of a broad space of dentine, while in the former the anterior valley still con- 

 tinues open internally. The posterior valleys of the posterior three premolars are 

 as deep externally as the corresponding portion of the anterior valleys. The basal 

 ridge of the three teeth just designated, envelopes the base of the postero-internal 

 lobes to a much greater extent than upon the preceding lobes, and in the specimen 

 under observation the first premolar presents an almost equilateral triangular tritu- 

 rating surface, possessing both internal lobes in a rudimentary state. Constituent 

 portions of the basal ridge connect the bases of the lobes together. The short ante- 

 rior valley remains open internally, but the external extremity only of the posterior 

 exists as a small trilateral pit of enamel. 



Inferior molars. — (Tab. ix., fig. 2.) The teeth preserved in the fragments of 

 lower jaws referred to belong all to the posterior four molars, and these do not differ 

 in their form from those corresponding to them in recent species of Rhinoceros. 



A basal ridge exists in all, but is obsolete on the inner side of the posterior three 

 molars and the outside of the hinder lobe of the same teeth, except the last. 



ADMEASUREMENTS. 



Length from occipital condyle to first premolar, . . . .141 



Distance from end of post-glcnoid tubercle to parietal crest, ... 6 



Height of face from alveolar border to supra-orbitar prominence, . . 5J 



Distance from occipital condyle to anterior margin of orbit, . . 10 J 



Breadth of skull at zygoma, ...... 8f 



" of forehead at supra-orbitar protuberances, ... 7 



Length of upper molar scries, . . . . . 7i 



