-546 



OREODON OF NEBRASKA. 



third molars, more frequently in the latter only than in both. Between the internal 

 lobes the ridges occasionally conjoin and form a common intervening tubercle. 



When the enamel summits of the lobes of the true molars are worn off, the 

 exposed dentinal surface of the outer lobes presents the form of W, or two crescents 

 continuous, that of the inner lobes of two distinct crescents. 



The crown of the fourth premolar is composed of two pyramidal lobes, like those 

 of the true molars but much larger. 



The crowns of the anterior three premolars are single, three-sided pyramids with 

 a pointed apex decreasing in size from the third to the first, and nearly alike in 

 their details. Their outer face is broad and condiform, less concave and more ver- 

 tical than in the succeeding teeth. The two inner sides are separated by a pro- 

 minent angle in the middle line. 



The postero-internal face is excavated into a broad cul-de-sac, the antero-internal 

 face into two smaller culs-de-sac. 



The true molars are implanted by four roots, the third and fourth premolars by 

 three roots ; two external and one internal, and the anterior two premolars each 

 by two roots. 



The inferior true molars (Tab. x., fig. 5 ; xiii., 3-G) have nearly the same size 

 and form of those of Cervus Virginianus. The outer lobes are less oblique in their 

 relative position to one another than in the Deer, and are broader at base externally, 

 but become more tapering towards the apex. Their inner face is concave, and 

 much more shallow than in the Deer, but gains in breadth what it loses in depth. 

 The crescentic summits of the outer lobes of each tooth at their contiguous extre- 

 mities, become continuous. 



Anteriorly and posteriorly the true molars possess a basal ridge, and between 

 the outer lobes of each tooth the latter constitutes a broad pyramidal heel. 



The internal faces of the inner lobes present three folds, as in the Deer, but the 

 posterior marginal fold is shorter, thicker, and more divergent backward from the 

 base. 



The fifth lobe of the posterior molar is more simple in form, and more distinct 

 from the adjoining lobes than in the Deer. In transverse section it is a broad 

 ellipse, and has an acute U-shaped summit. 



In the attrition to which the true molars are subjected, the acute enamelled sum- 

 mits of the pyramidal lobes soon give way to crescentic surfaces of exposed dentinal 

 substance, which gradually increase in breadth, or the crowns are worn down, until 

 finally the whole enamelled triturating surface is obliterated. The crowns of the 

 inferior three premolars are quite peculiar in their form. Exteriorly they bear 

 considerable resemblance to those of the Deer, but do not exhibit the deep vertical 

 depression so conspicuous in the teeth of the latter. Internally, they are much less 

 complex than in the animal just mentioned. 



Each premolar is constituted principally by a single broad pyramidal lobe, very 

 much longer than that of the true molars. The anterior portion of their outer side 

 is continuous obliquely inwards, so as to bring the anterior margin within the posi- 

 tion of the preceding tooth posteriorly. The summit separating the outer from the 

 inner side, is an acute ridge rising to a point. 



