DAKOTA AND NEBRASKA. 85 



increase in size from first to last, in each jaw, but the fourth or lateral incisor of the 

 lower jaw much exceeds all the others. 



The superior incisors with their fangs curve forward and downward, and their 

 crowns are directed downward. The inferior incisors are directed obliquely upward 

 and forward. 



The crowns of the upper incisors are ovoidal, convex externally, excavated inter- 

 nally, and have an acute rounded summit and lateral borders. The latter expand 

 abruptly at about the middle of the crown, forming, as in the Wolf, short lateral 

 offsets, which become continuous with an internal basal ridge. 



The crowns of the inferior incisors are rather trapezoidal in their outline, and they 

 have their external face more flat and their summit much broader than the upper 

 ones. The inner face is angularly convex in the intermediate teeth, and forms a pair 

 of inclined, slightly depressed planes, uniting in a median acute ridge. The lateral 

 acute borders form offsets at or below the middle, and unite posteriorly through 

 means of a strong basal ridge. 



The inferior lateral incisor is twice the size of the others, and, as in other 

 ruminants, is to be regarded as an incisiform canine. 



Tempoi'ary dentition. — The temporary dentition of Oreodon was probably arranged 

 according to the following formula : 



3—3 1 — 1 2—2 1 — 1 



In. ; c. : p. m. ; m. — = 30. 



4—4 1—1 2—2 1 — 1 



SjDecimens under examination contain the temporary molar teeth and the upper 

 canines, but no others. 



The upper temporary true molar has the same form as those of the permanent set, 

 but is smaller. 



The crown of the upper second temporary premolar is composed of three lobes like 

 those of the true molar j two behind and transverse, the other in front. It has the 

 appearance of the fourth permanent premolar conjoined with an anterior premolar. 

 The anterior lobe internally is connected with the adjacent part of the postero- 

 internal lobe by means of a festooned fold, enclosing between them a depression. 



The upper first temporary premolar resembles the corresponding permanent tooth 

 less well developed. 



The first permanent premolar of the upper jaw protruded after the deciduous teeth 

 behind, and appears not to have had a predecessor. 



The lower temporary true molar, as in other ruminants, possesses three pairs of 

 lobes, which hold the same relative position with one another and have the same 

 form as the pairs of lobes in the permanent true molars. 



The two lower temporary premolars closely resemble those of the permanent set. 



