90 RHINOCEROS. 
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 Fhinoceros Nebrascensis 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.—(P1. XIV.1,153; XV.5.) The upper molars bear a very great 
resemblance in form to those of Aceratheriuwm 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 anterior 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 upon the anterior valley; but the antero-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 Ehinoceros 
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 [?hinoceros 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 more minutely than may be considered essential. (XIV. 4-8.) 
The first premolar is only three-fourths the size of the others, and it is trilateral 
with the inner and posterior sides, forming a continuous convexity. The posterior 
