382 THE EOCEXE PERIOD. 



On my return home, September Ist, after an absence of three months, I found 

 that various parts of the skeleton of Periptychus^ have reached my museum. 

 On examination, I find that the astragalus of that genus fulfils the anticipa- 

 tion above expressed. It is without trochlea, and nearly resembles that of 

 Elephas. As it agrees nearly with that of Phenacodus in other respects, I 

 only separate it as a family from the Phenacodontidce. One other type re- 

 mains to be discovered which shall connect the Periptychidce and the hypo- 

 thetical Hyodonta, and that is a Taxeopod without a head to the astragalus — 

 unless, indeed, the "Hyodonta" should prove to have such a head. I think 

 the latter the less probable hypothesis, and hence retain the term Platyar- 

 thra for the hypothetical Taxeopod without trochlea or head of the astrag- 

 alus. 



These relations may be rendered clearer by the following diagram: 



Taxeopoda. 

 Condylarthra, Platyarthra.* 



Hyracoidea. \ 



Pboboscidea. Amblypoda. 



Hyodonta.* Pantodonta. 

 I Dinocerata. 



DiFLABTHBA. 



/ \ 



Perissodactyla. Artiodactyla. 



The preceding classification was first published in the Proceedings of 

 the American Philosophical Society, October, 1882, after having been read 

 at the Montreal meeting of the American Association for the Advancement 

 of Science, August 29, of the same year. 



CONDYLARTHRA. 



Cope, American Natnralist, 1881, p. 1018, November 29. 



In a paper on the "homologies and origin of the molar teeth of the 

 Mammalia Educabilia, published in March, 1874,' I ventured the generali- 



' Si'o Ariicricaii Nataraliat, October, 18s2. 



'Hypothetical. 



'Journal of the Acatlemy of Natural Sciences, Philadelphia. 



