Cope.] 



388 



[Oct. 7, 



The geological positions of these genera are as follows : 



The D1CHODONTID..E are allied to the Oreodontidse and Tragulidfe in the 

 simple form of the premolars, excepting the fourth in both jaws. These 

 resemble, according to Schlosser,*the milk teeth of other ruminants in the 

 genus Dichodon. For the present I associate with this European genus, 

 the American Agriochoerus and Coloreodon, in which the last lower pre- 

 molar resembles a true molar, and the last upper has two external Vs as 

 in a true molar. The structure of the feet in these genera is unknown, 

 but in Dichodon they are supposed to consist of the two median digits 

 only, which do not form a cannon bone.f It is safe to conclude that the 

 American forms do not possess a cannon bone, and as the presence or 

 absence of lateral digits is not always a family character, I leave them 

 provisionally in the DichodontidtB to which they are in any case nearly 

 allied. They agree also in the resemblance of the inferior canine to an 

 incisor tooth, but the first premolar is caniniform in the American genera, 

 which it is not in Dichodon. The genera differ as follows : 

 I. First inferior premolar caniniform. 



Premolars three Coloreodon Cope. 



Premolars four Agriochcerus Leidy. 



IL First inferior premolar not caniniform. 

 Premolars four Dichodon Owen. 



In this instance the genus of more modern type, Dichodon, is the oldest 

 in time (Upper Eocene), the other genera being Lower Miocene. These 

 types have been derived from some common ancestor of the family of 

 Oreodontidie as here defined. Dichodon may be a descendant of Lophi- 

 omeryx, and Agriochoerus be derived from an ancestor in common with 

 that genus and Dorcatherium (for diagram see under Oreodontidiie). 



The Tragulidje has a good many extinct genera, and the genus Tragu- 

 lus is represented by several living species. It is difficult to separate this 

 family from the Oreodoutidte, and the only character which appears to be 



* Beitriige z. Kenntniss d. Stammesgeschichte d. Hufthiere u. Versuch einer System der 

 Paar- u. Unpaarhufer. Morphologisches Jahrbuch, 1S86, p. 56. 

 t The Diplopus of Kowalevsky is supposed to be Dichodon, by Schlosser. 



