DAKOTA AND NEBRASKA. 81 



In the trituration to which the upper true molars were subjected in mastication, 

 the same steps were passed through as usual in living ruminants. The summits of 

 the anterior lobes suffered abrasion first and most; the internal lobes more than the 

 external ones, but subsequently the wearing appeared more equalized. When tlie 

 dentine is first exposed on the outer lobes, the surface presents the form of a pair of 

 crescents confluent where contiguous. At the same time the inner lobes exhibit 

 broader crescents of the exposed dentine. As attrition proceeded, the dentinal cres- 

 cents of the inner lobes liltewise became continuous, while those of the outer lobes 

 became wider*. Subsequently the inner and outer crescents became conjoined, 

 leaving upon the dentinal surface of each tooth a pair of central crescentic enamel 

 islets, composed of a small portion of the external face of the inner lobes and a larger 

 portion of the internal face of the outer lobes. The enamel islets next disappeared, 

 leaving upon the teeth broad quadrate dentinal surfaces bordered by enamel. 



The crown of the fourth upper premolar is composed of a pair of crescentoid demi- 

 conoidal lobes, like those of the true molars, but larger, as is the case with all the 

 upper premolars of ordinary ruminants. Its outer face is cordiform and transversely 

 concave, with a slight median ridge. 



The anterior three upper premolars are constructed after the same plan. They 

 decrease, successively in size and in the degree of development of their details of 

 form, from the third to the first of the series. Their crown is a ti'ilateral pyramid, 

 with a pointed apex and a broad external cordiform surface. The narrower internal 

 surfaces appear as triangular inclined planes, separated by a median acute ridge 

 extending from the point to the base of the crown. The anterior of the internal 

 surfaces forms at the base a pair of shallow pouches, defined by a double festoon. 

 The posterior of the same surfaces forms a single and larger pouch at the base of the 

 crown, included by a single and thicker festoon. This latter in the third premolar 

 almost assumes the dignity of an additional lobe to the cx'own, resembling the internal 

 lobes of the true molars. 



The outer cordiform surfaces of the anterior three upper premolars become succes- 

 sively less concave, from behind forward, apparently from a gradual expansion of 

 the slight median ridge, so that the first of the series is almost convex. 



In some instances the fourth upper premolar exhibits a ridge or process dividing 

 off the fore extremity of the crescentic interspace of the two constituent lobes of the 

 crown. 



Occasionally the antero-internal surface of the crown of one or more of tlie anterior 

 three upper premolars forms three pouches at the base of the crown, and occasionally 

 also only a single pouch. 



The crown of the fourth premolar in wearing passed through the same steps as a 

 corresponding pair of lobes of the true molars. In the abrasion of the other pre- 



11 



