GREGORY: NOrHARCTVS, AN AMERICAN EOCENE PRIMATE 



149 



Summary of Occlusal Relations of the Upper and Lower Teeth 



Referring to the writer's conclusions (1915, pp. 422, 423) regarding the supposed correlation between 

 differences in the excursion of the mandible and concomitant differences in the upper and lower molars 

 Dr. Stehlin asserts {op. cit., p. 1538) that: "Gregory nimmt in sehr zuversichtlicher Weise Stellung zu 

 diesen Fragen." He also said in this connection that: "Im iibrigen scheinen mir die Ausfiihrungen 

 Gregorys nicht nur rein hypothetisch, sondern sehr anfechtbar zu sein" (p. 1539). 



In reply, it may be proper to state that the conclusions under consideration, although perhaps too 

 briefly stated, were not hasty outgivings of the moment but were one of the incidental results of long 

 continued study on the mechanical interrelations of the parts of the upper and lower teeth in many recent 

 and fossil mammals, on the areas and attachments of the jaw muscles, and on the movements of the 

 mandible in mastication, it having been found by experience that all these facts were closely interrelated 

 and that they often contribute to a better understanding of the probable origin and evolution of the 

 dentition and of the skull as a whole. In view of the abundant material described and figured in the 

 present paper and summarized above (pages 133-148) the following may be taken not as theoretical 

 deductions but as literal facts. 



(1) In the earlier Notharctinse, between the protocone and the incipient pseudohypocone of the 

 upper molars, there is often a notch-like facet of wear, caused by the attrition of the entoconid 

 of a lower molar. (Fig. 39.) 



(2) On the posterointernal face of the incipient pseudohypocone of these forms there is an extended 

 facet of wear caused by the attrition of the paraconid of a lower molar. (Fig. 39.) 



(3) The pseudohypocones of the upper molars and the entoconids of the upper molars are but poorly 

 developed in the earliest Notharctinse, become gradually larger in those of intermediate age 

 and culininate in the very large pseudohypocones and entoconids of the latest and most pro- 

 gressive species Nothardus crassus. (Fig. 38.) 



(4) A precisely analogous case is supplied by the Eocene and Oligocene titanotheres, in which the 

 posterointernal cusp of the upper premolars develops pari passu with the entoconid of the lower 

 premolars; the tip of the entoconid likewise sweeps across the lingual face of the upper tooth 

 in the region of the constriction separating the "pseudohypocone" from the protocone (1915, 

 p. 423). 



(5) In Adapis parisiensis, on the contrary, the posterointernal cusp (true hypocone), which is an 

 upgrowth of the cingulum, has no direct relation at all with the entoconids of the lower molars; 

 it rather protrudes into the space between adjacent upper teeth and above the basin of the 

 trigonids of the lower molars, as it does in all normal mammalian dentitions (Plate XLI). 



(6) The generally retarded and internally placed entoconids in the Adapinte do not articulate with 

 the region of the pseudohypocone which never develops; they have, on the contrary, a func- 

 tional relation with the whole of the posterointernal slope of the protocones of the upper molars 

 since they help to press the food against them (Plate XLI). 



(7) As a result of the foregoing facts it follows that Dr. Stehlin's citation (p. 1539) of the variable 

 development of the true hypocones and of the entoconids in the Adapina by no means disproves 

 the thesis of the writer that a high pseudohypocone in the Notharctinre was correlated with a 

 progressive entoconid (or vice versa), a fact which can be determined by actually "fitting the 

 upper and lower teeth together," as originally stated (1915, p. 422). 



