DAKOTA AND NEBRASKA. 83 



the crown, both internally and externally, frequently more or less associated by a 

 transverse basal ridge at the posterior part of the crown. 



The structural arrangement thus described of the inferior premolars is subject 

 to considerable variation, arising from difference in degree of development. In 

 comparing the premolars with the true molars the outer portion of the crowns of the 

 former appears to be homologous with the outer pair of lobes of the latter, and the 

 postero-internal tubercle of a premolar appears to correspond with the antero-interual 

 lobe of a true molar. 



In tracing the effects of mastication upon the inferior premolars, it is noticed that 

 the dentine is first exposed on their posterior slope, and subsequently on the anterior 

 slope, while greatly widening in the former position. At an intermediate stage the 

 exposed dentinal tract posteriorly encloses an enamel islet or lake. 



In some specimens more than in others, the anterior pair of premolars appear 

 somewhat crowded in position, and are inserted obliquely, crossing the alveolar 

 border, so that the first one at its back part is situated externally to the second one, 

 and its fore part is situated internally to the contiguous canine tooth. 



All the molar teeth of Oreodon are inserted with the same number of fancis and in 

 the same relative position as in ordinary existing ruminants. 



The possession of well developed canine teeth in both jaws is one of the remarkable 

 characters of Oreodon. The form of these teeth is peculiar, and among recent animals 

 most resemble those of suilline animals, though comparatively of small size. They 

 vary in size in different skulls, but usually are best developed in the largest skulls, 

 which in many cases most probably indicate the male animal. 



The superior canine is separated by a short hiatus from the contiguous premolar 

 tooth, sufficient to accommodate the point of the inferior canine. It curves from the 

 bottom of its alveolus, above the position of the interval of the first pair of premolars, 

 forward and downward with a slight inclination outward. The fang is curved 

 trihedral, with rounded borders. The crown protrudes downward and outward, and 

 when the mouth is closed it occupies a position in front of and in contact with the 

 fore part of the crown of the inferior canine. 



The shape of the crown of the upper canine, in the unworn or slightly worn con- 

 dition, is that of a sharp pointed, trilateral pyramid, with nearly equal sides and 

 almost straight, or with the slightest curvature. One of its faces is directed out- 

 wardly, a second inwardly, and the third posteriorly. The outer face is slightly 

 convex, with a feeble wide median groove. The inner face is also slightly convex, 

 and is provided with a stout median ridge, narrowing away and disappearing towards 

 the point of the tooth. The posterior face is slightly concave, but soon becomes a 

 nearly vertical plane from attrition against the inferior canine. The anterior border 



