THE PALEONTOLOGIC RECORD 475 
those of the underlying beds except that some of them have well-de- 
veloped lachrymal vacuities while others have none. Another new Taco 
also makes its appearance suddenly, and in great abundance, in the 
genus Promerycocherus—structurally derivable perhaps from some 
of the older oreodons, but not connected with them by intergrada- 
tions. Agriochwrus has disappeared. In the Upper Rosebud the 
Oreodon-Merychyus 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- 
cocherus disappears entirely and is replaced by a very distinct and 
more advanced genus Merycocherus. 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 Merycocherus in one locality with Merychyus (both repre- 
sented by more specialized species) ; in another locality it contains 
instead, Promerycocherus with Ticholeptus (allied to Merychyus) ; in a 
third is found the most highly specialized member of the Merycochwrus 
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 
