OREODON. 41 



The inner lobes internally are less prominent in the middle luie ; their posterior 

 marginal fold is less developed, is shorter, and more divergent; and their external 

 face is more convex. (III. 4.) 



The outer lobes are less angular externally, and are more tapering from their base. 



Finally, the tubercle between the bottom of the outer lobes is not conoidal, but 

 it appears as a transverse talon with an external angular notch. 



The posterior lobe of the last true molar is relatively larger than in the Deer, 

 and is more elliptical in transverse section. (III. 5, 6.) 



In the attrition to which the inferior true molars are subjected in mastication, 

 crescentic surfaces of dentinal substance are exposed by the removal of the origi- 

 nally acute enamel summits of the lobes. As the wearing progresses, the dentinal 

 crescents increase in breadth ; most rapidly upon the outer lobes until the abrasion 

 reaches the bottom of the inner faces of these ; at which period the outer dentinal 

 crescents are considerably below the inner ones, and are twice as broad. At the 

 next stage, a small portion of the internal face of the outer lobes, in union with a 

 larger portion of the external face of the inner lobes, is left between each trans- 

 verse pair of lobes as a crescentic islet of enamel upon a broad surface of dentine. 

 The external portion of the islet is next obliterated, and is soon followed by the 

 remainder or internal portion. At a late period, the upper surface of the external 

 portions of the basal ridge is worn away by the apices of the outer lobes of the 

 superior molars, so as to leave shallow pits in the dentine. (VI. 5, 8.) 



The inferior premolars exhibit more peculiarity in comparison with the true 

 molars, than do the anterior three upper premolars. (II. 1, 3 ; III. 3, 4 ; VI. 4, 5, 8, 9.) 



In the Deer, it is easy to trace in the lower premolars, the constituent lobes of 

 the true molars. Thus, in the third premolar the anterior pair of lobes are very 

 like the corresponding pair in the true molars, but the posterior pair have under- 

 gone modification in size, form, and position. In the second premolar, the antero- 

 internal lobe of the true molars appears to be represented by the anterior double 

 fold, their corresponding outer lobe by the succeeding largest fold, and their poste- 

 rior pair of lobes by the two hinder folds. In the first premolar, all of the four 

 lobes may be traced as in the second premolar; but all, save the homologue of the 

 antero-external lobe, are reduced to their most rudimentary condition, and in some 

 individuals are entirely obsolete. 



The plurality of lobes of the true molars is much less readily distinguishable 

 in the premolars of Oreodon than in those of the Deer ; nevertheless, their gradual 

 disappearance may be traced. 



The premolars decrease in size from the last to the first, and each presents an 

 antero-posteriorly broad pyramidal crown. The transverse section of the crowns of 

 the posterior two at their base is an isosceles triangle with the apex directed for- 

 ward; while the section of the crown of the anterior at the same position is elliptical. 



Externally they are all prominent at the middle and are convex transversely, and 

 the posterior half of the surface is directed outward ; while the anterior portion 

 presents forward and outward. The first premolar is simply convex externally; the 

 second presents a slight fold at the posterior part of the external surface ; and the 

 third is depressed posterior to the median prominence of the same surface. Postero- 



