78 THE PALEONTOLOGIC RECORD 



those of the underlying beds except that some of them have well-de- 

 veloped lachrymal vacuities while others have none. Another new race 

 also makes its appearance suddenly, and in great abundance, in the 

 genus Promerycochcerus structurally derivable perhaps from some 

 of the older oreodons, but not connected with them by intergrada- 

 tions. Agriochcerus has disappeared. In the Upper Rosebud the 

 Oreodon-Merycliyus phylum shows a distinct and marked advance in 

 the length of the crowns of the teeth; lachrymal vacuities are always 

 present, the feet are decidedly more compact and elongate. Promery- 

 cochoerus disappears entirely and is replaced by a very distinct and 

 more advanced genus Merycochoerus. The Leptauchenia series has dis- 

 appeared temporarily, to re-appear in the Middle Miocene in a more 

 specialized genus, Cyclopidius, the last known member of this race. 



The Middle Miocene (which should follow the Upper Rosebud) is 

 unrepresented at the locality under consideration (Pine Ridge, South 

 Dakota), but elsewhere overlies beds with an equivalent fauna, and 

 contains Merycochoerus in one locality with Merychyus (both repre- 

 sented by more specialized species) ; in another locality it contains 

 instead, Promerycochcerus with Ticholeptus (allied to Merychyus) ; in a 

 third is found the most highly specialized member of the Merycochoerus 

 line, Pronomotherium. In the Upper Miocene and Lower Pliocene the 

 oreodonts become much scarcer, and the skulls and skeletons are known 

 only in two or three instances. Pronomotherium certainly occurs in 

 Montana; in Nebraska the Merychyi are more advanced in dentition, 

 belonging to a distinct subgenus Metoreodon; but whether the skulls 

 and skeletons are equally different we do not yet know, nor are we in 

 a position to say whether the change is gradual or saltatory. 



But the sum of results in regard to the changes from one stage to 

 another in this best known group of fossil mammals is either that the 

 changes are abrupt, constituting clean-cut faunal divisions marked by 

 the sudden appearance in abundance of a more advanced stage; or else 

 that the new form replaces the older one little by little, but on the 

 whole can not be fairly said to be gradually converted into it by 

 infinitesimal gradations. 



This general observation applies, in my opinion, equally well to any 

 abundant group of fossil vertebrates whose phylogeny is sufficiently 

 known to make them worth considering. 



If, therefore, we consider that the record is continuous where there 

 is no apparent stratigraphic break, and that the known record really 

 represents what was going on over the entire continent of North 

 America, I do not see that we can fairly escape from the conclusion that 

 new species, new genera and even larger groups have appeared by salta- 

 tory evolution, not by continuous development. 



But and here lies the crux of the whole question we have no 



