232 ON THE EXTINCT MAMMALIA OF 



base, and in the posterior lobe being provided near its middle with a prominent crest 

 projecting forward into the transverse valley of the crown. An isolated antero- 

 internal lobe of a tooth in advance, as in the preceding case, also presents a deeper 

 constriction than in the corresponding teeth of B. occidentalis. The antero-postero 

 diameter of the perfect crown of the last molar measures fifteen and a half lines ; the 

 transverse diameter eighteen lines. 



HYRACODON. 



The genus Hyracodon is founded on fossil remains formerly referred to a species of 

 Rhinoceros, with the name of R. nehrascensis. It is characterized by having molar 

 teeth constituted like those oi Rhinoceros, Hyrax, and the extinct Aceratherium, but is 

 further provided with canine teeth and a full series of incisors in both jaws. As in 

 the last-named genera, it is also destitute of horns. 



Hyracodon nebrascensis. 



Many fossil specimens of this species have been brought from the Mauvaises Terres 

 of White River, Dakota, where they appear to be equally abundant with the remains 

 of Rhinoceros occidentalis. The animal was about two-thirds the size of the latter, 

 and less than half that of the living Indian Rhinoceros. 



Besides numerous fragments of jaws with teeth, I have had the opportunity of 

 examining the greater portions of six mutilated skulls, from which I am enabled to 

 give a nearly complete account of the most characteristic part of the skeleton. 



The perfect skull of Hijracodo^i nebrascensis has nearly the same form as that of R. 

 occidentalis, and, as in this, its upper outline is nearly straight, though sloping 

 moderately in front. 



Views of the greater portions of several skulls, including the molar dentition, of 

 Hyracodon nebrascensis, are given in plates xiv and xv of the Ancient Fauna of Ne- 

 braska, but want of sufficient means has prevented the representation of additional 

 and in some instances better specimens in the present work. 



Lateral view of the skull. — The temporal fossa of Hyracodon nehrascensis appears 

 proportionately not so long as in Rhinoceros occidentalis, though it extends a greater 

 distance along and near the median line of the cranium. The temporal surface is 

 convex, and rises upon a sagittal crest extending two-thirds of its length. Posteriorly 

 it exhibits several large venous foramina. 



The zygoma pursues nearly the same course as in the Indian Rhinoceros. Its 

 hinder root springs from about the middle of the lower boundary of the temporal 

 fossa, and has its upper surface convex fore and aft. 



