OllEODON OF NEBRASKA. 547 



The middle jDortion of the inner side of the third premolar is constituted by a 

 pyramidal process, nearly as high as the apex of the principal lobe, with which it 

 is continuous. Posteriorly, the inner side of this tooth has a quadrilateral cul-de- 

 sac, bounded internally by a short pyramidal tubercle. The surface of the anterior 

 portion of the inner side is depressed. 



The second premolar internallj^ presents a ridge descending from its apex a little 

 backwards, and expanding along the base of the crown. Anteriorly and posteriorly 

 to the ridge, the surface is depressed, and the ridge sends an ollset backwards, which 

 terminates abruptly about the centre of the posterior depression. 



The first premolar presents a ridge similar to that existing in the second, but 

 much less developed, and without the posterior offset. 



The inferior molars are inserted by two roots, except the last, which has three, 

 like in existing Ruminants. 



The canine teeth of Oreodon are (^uite peculiar in form, and are about the 

 same length in both jaws. 



The superior canines curve forward, downward, and in a less degree outward. 

 Those of the male are a little more robust, and are directed more externally than 

 in the female. Their crown is a trihedral pyramid, with acute margins and a 

 pointed summit. The sides arc nearly equal, being posterior, antero-internal, and 

 external, and are smooth. The posterior side is opposed to the anterior margin of 

 the corresponding tooth below. 



The inferior canines are straight, occupy a position posterior to those above, and 

 take a course obliquely upwards, forwards, and outwards. The crown is a broad, 

 transversely compressed pyramid, with an inner and outer convex side ; an anterior 

 trenchant margin, opposed to the posterior surface of the canine above, a posterior 

 trenchant margin, and a pointed summit. 



Of the three superior incisors on each side, the internal is the smallest, the other 

 two being nearly equal in size. The outer fiice of their crowns is convex, and 

 nearly ovoid in outline. 



Of the inferior incisors, the internal of each side is also the smallest ; the succeed- 

 ing two are nearly equal in size, and the lateral, or fourth incisor, is a fifth larger 

 than those last mentioned. The outer face of the three mid incisors is convex and 

 oblong quadrilateral ; and that of the fourth incisor is also convex, but is more tra- 

 pezoidal in its form. 



Temporcmj dentition.— T\\q deciduous teeth of Oreodon, so far as can be ascer- 

 tained, are as follow : in. - can. i=l p. m. —- m ^— ^ 



'< 1—1 J- 2—2 ■ 1—1* 



The first permanent premolar of the upper jaw appears to have no temporary 

 predecessor. The superior true deciduous molar is exactly like the permanent true 

 molars in form, and is about one-sixth smaller. 



The crown of the second premolar is composed of three pyramidal lobes ; two 

 posterior and transverse, the other anterior and opposite the former. 



The first premolar in form is very nearly like its successor. 



In the lower jaw, the deciduous true molar has six pyramidal lobes in three 

 transverse pairs, as in existing Ruminants, etc., and the true premolars have the 

 same form as their permanent successors. * 



