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ROYAL SOCIETY OF CANADA 



loph, but is curtailed at this point, in consequence of which there is 

 only a slight indication of the fonnation of a posterior valley (postsinus) 

 that in H. nebrascensis has reached a more advanced stage. 



The order of premolar transformation in H. priscidens is appar- 

 ently an exception to the usual metamorphosis of the Hyracodont pre- 

 molars, which, as stated by Osborn in his memoir on "The 'Extinct 

 Ehinoceroses," 1898, p. 90, is presented in three successive stages of 

 evolution toward the molar pattern "in the second, third and fourth pre- 

 molars, the last premolar (p'*) being the most advanced. In H. pri- 

 scidens the fourth premolar is the least advanced as regards 'the relation 

 of the lophs to each other, although in other respects, viz., in the pre- 

 sence of secondary crest folds ("crista," " antecrochet " and "crochet") 

 a decided advance has been made, and it may be considered in this re- 

 gard as more progressive than p^, p^ and p^. 



Measurements. 



^ The measurements of the teeth of H. planiceps and H. nebrascensis are 

 taken from the " Preliminary account of tihe Fossil Mammals from the White 

 river formation, contained in the museum of Comparative Zoology," by W. B. 

 Scott and Henry F. Osborn, 1837, p. 171, Bull. Mu3. Oamp. Zool., Harvard 

 College. 



