216 Annals of the Carnegie Museum. 



throughout the region, occasionally revealing slight indications of 

 non-conformity, as has been explained by the writer in earlier pub- 

 lications. Near the top of the Lower Harrison beds is found Pro- 

 merycochcerus vantasselensis, while in the Upper Harrison beds Mery- 

 cochcerus is found in considerable numbers. We have seen that the 

 anatomical differences between the two genera are too great to allow 

 us to regard the latter as descended from the former, unless we admit 

 that there was a considerable lapse of time between the deposition 

 of the two horizons. The Upper Harrison beds consequently must 

 be regarded as of much later origin than the Lower, possibly repre- 

 senting the base of the Middle Miocene. This would give time enough 

 for the evolutionary changes which are required under the hypothesis 

 that Merycochcerus is descended from Pr ornery cochosrus. The litho- 

 logical differences in the two horizons are indeed slight, but, such as 

 they are, they tend to support the idea of a break or hiatus between 

 the two horizons. 



The general similarity of the lithological features of the two horizons 

 on the other hand seems to indicate that these sediments were laid 

 down under somewhat similar conditions, and that their source was 

 the same. If the view that there was no appreciable interval between 

 the deposition of the Lower and Upper Harrison beds should be suc- 

 cessfully maintained, I am puzzled to conceive in what way the general 

 conditions of life should have been so altered in such a limited geo- 

 graphical locality and in a comparatively short geological period of 

 time in such a way as to cause these animals to undergo the mutations, 

 which we are forced to admit must have occurred in order that we 

 may connect Promerycochcenis and Merycochcerus as members of one 

 phylogenetic stem. 



In his remarkable work, "The Age of Mammals," Professor Osborn 

 is inclined to attribute the sediments of this region, which include the 

 Gering, the Monroe Creek and the Lower Harrison beds, and which 

 immediately overlie the Leptauchenia beds, to the Upper Oligocene. 

 One of his statements is as follows: (/. c, p. 232) "It would appear 

 this mammalian assemblage of the Upper John Day, Lower Arikaree, 

 Lower Harrison, and Lower Rosebud is still characteristically Oligo- 

 cene rather than Miocene." From the view thus expressed the writer 

 is inclined to dissent. In my opinion there is no reason to regard the 

 faunal differences between the Lower and the Upper Harrison beds as 

 being any greater than, if as great as, those between the Leptauchenia 

 beds and the Gering and Monroe Creek horizons. Would it not be 



