59 



contemporary selenodont artiodactyla, the superior incisors 

 persisted. I have, however, discovered that these teeth per- 

 sisted in the Loup-Fork genus Protolabis during adult life. 

 I have found that one (the second) of these teeth in Procamelus 

 occidentalis persisted, without being protruded from the 

 alveolus, until nearly adult age. 



" In genera (V.-.,the bunodont artiodactyla) where the inci- 

 sors are normally developed, they appear at about the same 

 time with the other teeth, and continue to develop to func- 

 tional completeness. This development is retarded in Pro- 

 tolabis, since they are not so matured as to remain fixed 

 throughout life in their alveoli. 



" In Procamelus, the retardation is still greater, since the 

 first incisor reaches very small dimensions, and is, with its 

 alveolus, early removed ; while the second incisor only grows 

 large enough, and for a sufficient time, to occupy a shallow 

 alveolus, without extending beyond it. In the first incisor 

 the process of retardation has reached its necessary termin- 

 ation, i.e., atrophy or extinction ; while in the existing Came- 

 lidce the second incisor also has disappeared the same way. 

 In ruminants other than Camelidce, the third or external in- 

 cisor has undergone the same process, while in the Bovidce the 

 canines also have been retarded in development, down to 

 atrophy." 



Professor Cope continuing the discussion of the teeth ol 

 these types, generalizes : " From these and many analogous 

 cases, the general law may be deduced that identical modifi- 

 cations of structure, constituting evolution of types, have supervenea 

 on distinct lines of descent ? " 



This summary of Professor Cope's conclusions has been 

 necessary to show clearly in this series the probable place of 

 the eocene genus. 



From the shape of tne premaxillaries and maxillaries, and 

 the form and position of the teeth the camelline affinities have 

 been indicated. Future research must be relied on to throw 

 light upon the structure of the limbs and the axial skele- 

 ton ; as it is from these that the conclusive proof must be 

 deduced that Ithygrammodon is the ancestor of the camels, 

 and that Poebrotherium is the second link in the chain, instead 

 of the progenitor. 



