TITANOTHEEIUBI. 73 



The triturating summits of the lobes present more or less broad crescentic sur- 

 faces of exposed dentine, bordered by enamel, with the horns rising to the inner 

 side of the teeth, and there becoming confluent and forming simple conoidal promi- 

 nences. The enamel spaces embraced by the horns of the crescentic summits of 

 the lobes do not slope towards the base of the teeth internally, as is represented to 

 be the case in the figures of the corresponding teeth of PalaeoiJierium, but they form 

 deltoidal concavities, which are nearly on the same level with the dentinal cres- 

 cents, and are bounded internally by a thiols obtuse border; open, however, at the 

 middle to the bottom of the concavities. 



The third lobe of the last molar is smaller than those in advance, and resembles 

 one of them atrophied at its posterior half. The external basal ridge of this tooth 

 ceases upon the third lobe just before reaching its posterior surface; but upon this 

 internally a small portion is developed. 



The enamel of the teeth is rugose, and is most so externally, in which position 

 it also presents a very uniform series of transverse striae. At the triturating sur- 

 face of the teeth externally, where the enamel is thickest, it measures one line and 

 two thirds. 



The measurements of the teeth in the fragments of jaw just described, are as 

 follows : — 



ANTEEO-POSTERIOU. TRANSTEKSE. 



luclies. Lines. Inches. Lines. 



Last molar 4 6 1 10 



Second true molar ........ 3 3 2 



First true molar 2 8 1 10 



In the collection of Dr. Owen, there is preserved a portion of the left side of a 

 lower jaw (PL XVI., Figs. 2, 3) containing true molars exactly lilse those just de- 

 scribed, and the fangs of the preceding two premolars. Accompanying this specimen, 

 and probably derived from the same individual skeleton, there are also the crown of 

 the second or third left lower premolar, the crown of a lower canine, and fragments 

 of two upper molars. The fragment of lower jaw, before it became infiltrated with 

 mineral matter, was very much crushed, and at present it is more light and friable 

 than any other of the specimens of fossils which have been brought from Nebraska. 

 It is considerably smaller than the corresponding part of the bone in Dr. Prout's 

 specimens, measuring nine and a half inches along the series of true molars, five 

 and a quarter inches in depth below the last molar, and an inch and a quarter in 

 thickness below the first true molar. Two inches posterior to the last molar it is 

 seven and a quarter inches in depth. Its form closely corresponds with that of 

 Dr. Prout's specimens, as does also the form of the teeth contained in it, except 

 that their basal ridge is not of uniform depth, but gradually rises in a pyramidal 

 manner, and becomes thinner from between the lobes to their most prominent ex- 

 ternal part. 



The teeth are more worn than in the specimens of Dr. Prout, and their enamel 

 presents the same appearance, but in the same position is a third of a line less in 

 thickness. 



The measurements of the true molars are as follows : — 



