2 2 Bulletin Attierican Museu7n of Natural History. [Vol. XII, 



Basin of the Great Plains. — The precise relations of the fossil- 

 iferous horizons in different parts of this wide area stand in need 

 of further study. The deposits are coextensive with the Neocene 

 and Quaternary, but in no place has a continuous section been 

 observed, and the series of fossiliferous horizons is still more 

 incomplete. 



The following table is an attempt at correlation of a number 

 of typical sections with the section published by Hayden and 

 Leidy thirty years ago. The proper identification of these di- 

 visions has been prevented by a confusion as to the fauna properly 

 belonging to Horizon D. The descriptions given by Hayden of 

 the character of the rocks in the several divisions of his section 

 agree accurately with the corresponding divisions observed by 

 the writer in Colorado. But the fauna which Hayden and Leidy 

 ascribe to Horizon D was found by the writer in Horizon C, 

 while D belonged stratigraphically and faunally to the Upper 

 Miocene. The fauna which Leidy ascribes to (7 is a mixture of 

 Oreodon and Protoceras Beds species. 



The Middle Miocene does not appear to be represented in any 

 of the sections. The faunal break is least serious in Colorado 

 and Oregon, where a number of genera pass through without 

 more than specific change. Such are, in Colorado, Anchippus, 

 Aceratherium (sensu stricto), Alerycoc/itvrus, ? Canis. In Oregon, 

 Mesohippus and probably others. 



Another break occurs in the Pliocene, partly if not completely 

 bridged in Texas, but tolerably well marked elsewhere. 



It is probable that the Protoceras Beds overlap to some extent 

 on the Lower Miocene John Day. Further study of both and 

 determination of the exact horizons of different species will be 

 necessary to find the extent of the overlap. At present it seems 

 that some lines of descent are more advanced in one basin, some 

 in the other. The Rhinoceroses seem to have run into Dicera- 

 therium in the John Day, while continuing the line towards 

 Aphelops in the Plains. The Camels are more advanced in the 

 western basin, but the Horses are persistently primitive, while 

 Anchippus had already appeared in the Leptauchenia Beds. 

 Side branches of Canidae appear in both basins, while the 

 more direct line shows little difference in age between the two. 

 Merycochoerus and Eporeodon appear in both, but the relations of 



