90 RIIIN(3CER0S. 



The anterior mental foramen occupies a position near the base of the bone below 

 the hinder fang of the second molar of the remaining series. In advance of it, on 

 nearly the same line, are two other and smaller foramina of the same kind. 



A portion of the ramus shows this to have been thin and deeply excavated 

 internally, as in the Tapir. The posterior mental foramen is large, and placed 

 about one inch behind the last molar tooth. 



Dentition. — Except the first inferior molar tooth, which is shed at an early 

 period, the entire series of permanent molar teeth in Ehinoceros Nehrascensis is 

 retained to a late period of life, as is indicated by the specimen of a skull of a 

 very old individual in the collection of Dr. Owen, in which, although the crowns 

 are almost completely worn away, yet the whole number remains. 



From minute fragments of fangs of an upper and lower incisor existing in two 

 of the adult specimens under investigation, we are satisfied of the existence of 

 these teeth permanently, but the number we have no means of ascertaining. 



Superior Molars. — (PI. XIV. 1, 13; XV. 3.) The upper molars bear a verj' great 

 resemblance in form to those of Aceratlierium incisivum ; and they possess a basal 

 ridge all round except at the inner side of the bases of the internal lobes of the 

 true molars, and where it has been obliterated by pressure from the teeth in contact. 



The outer surface of the true molars is broad and slightly depressed at the 

 middle, and at the anterior fifth forms an abrupt fold, as in all other species of 

 Rhinoceros. 



The last molar exhibits a disposition to the development of a posterior valley, 

 or rather a separation, as in the other molars, of the postero-internal and external 

 lobes. The anteiior valley of this tooth is almost as deep as the crown, is nearly 

 level at bottom, and is bounded at its entrance by a mammillary eminence, which 

 is a portion of the basal ridge. The hinder lobe is quite simple, and exhibits no 

 tendency to encroach uj)on the anterior valley ; but the antex-o-internal lobe at its 

 middle posteriorly protrudes considerably into the latter. 



The inner lobes of the true molars in advance expand gradually to their base, 

 are impressed anteriorly, and protrude into the valleys about their middle poste- 

 riorly. The valleys are of equal depth at their outer extremities or termination, 

 and the principal ones, except in the penultimate tooth in one specimen in which 

 the bottom throughout is nearly uniform, deepen towards their entrance, so that in 

 the trituration to which the teeth are subjected in mastication, as in Rhinoceros 

 occidentalis and Aceratherium incisivum, they become obliterated from without in- 

 wardly. The entrance of the anterior or principal valleys in the fifth and sixth 

 molars is not obstructed by the existence of a constituent portion of the basal 

 ridge, as in Rhinoceros occidentalis. 



A small fragment of an upper jaw, presented to the Academy of Natural Sci- 

 ences by Mr. Alexander Culbertson, contained the crowns of the four premolars 

 entirely concealed within the bone. These, having been divested of their hard 

 envelop, are remarkable for their state of preservation and beauty, and lead me 

 to describe them moi^e minutely than may be considered essential. (XIV. 4-8.) 



The first premolar is only three-fourths tlie size of the others, and it is trilateral 

 with the inner and posterior sides, forming a continuous convexity. The ^wsterior 



