DAKOTA AND NEBRASKA. 235 



The hard palate of Hyrcwodon nehrascensis is strongly arched, though not to the 

 same extent it is in Rhinoceros occidentalis. It is proportionately much broader and 

 less convergent anteriorly than it appears to be in the latter. The inter-palatine 

 notch extends as far forward as the posterior third of the penultimate molar tooth. 



Lower jaw. — The lower jaw of Hyracodon nehrascensis presents intei*mediate 

 characters to that of the Rhinoceroses and Tapirs. 



The body of the lower jaw and its base, corresponding with the position of the 

 molar series, resemble the same part in the Indian Rhinoceros. The anterior ex- 

 tremity of the jaw resembles the corresponding portion of that of the Tapir. The 

 chin presents a similar mode of construction, and likewise the alveolar border 

 for the reception of an unbroken arch of canines and incisors. The symphysis is two 

 inches in length. There appear to exist usually three mental foramina, in a row in 

 advance of the fourth premolar tooth, extending as far as the hiatus in front of the 

 molar series. 



The back part of the jaw is comparatively high. The coronoid process is pro- 

 portionately longer and wider than in the Indian Rhinoceros, and its apex is elevated 

 above the base of the jaw a distance exceeding two-thirds the length of the latter. 

 The fossa on the outer surface of the ramus, below the coronoid process, is a much 

 larger and deeper depression relatively than in R. occidentalis. The notch back of 

 the coronoid process is not so wide as in the latter. 



Dentition. — The usual number of teeth in the unicorn Rhinoceroses is seven molars 

 and two incisors on each side of both jaws. The two-horned Rhinoceroses in the 

 adult condition have no incisors. The extinct genus Aceratherium possessed the 

 same number of teeth as the unicorn Rhinoceroses, but the incisors were larger. 

 Hyrajx, in the adult condition, has one incisor above and two below on each side. 



The dental formula of Hyracodon nehrascensis is as follows : 



3—3 1—1 4—4 3—3 



Incisors ; c. — ; p. m. — ; t. m. — :=44. 



3—3 1—1 4—4 3—3 



Prof Owen observes in his Odontography, p. 589, that Dr. Falconer informed him 

 that there are six incisors in both jaws of one of the extinct species oi Rhinoceros 

 from the Himalayas, but he makes no reference to canine teeth, so that it is probable 

 none existed. 



Hyracodon therefore, while possessing the usual number of molars of Rhinoceros, 

 etc., together with the unusual number of incisors of an extinct Himalayan species, 

 in addition also had canines, thus presenting a dental formula like that of the Tapir, 

 PalcBotherium, etc. 



The molar teeth in both jaws have the same relative position and general constitu- 

 tion as in recent species of Rhinoceros. The canines and incisors together in both 



