210 PROCEEDINGS OF THE PALEOXTOLOGICAL SOCIETY 



be extremely variable and the species do not appear to be good horizon- 

 markers. 



A discussion followed concerning the standards of species distinction 

 and methods of description among fossil mammals, in which the author, 

 Dr. TroxelL Dr. Thorpe, Professor Lull, Dr. W. D. Matthew, and Dr. 

 P. Hay participated. 



In the next, paper the author proposed to transfer the Harrison and 

 Rosebud group of formations of the Great Plains to the Oligocene, corre- 

 lating them with the John Day formation of Oregon, and included in the 

 Miocene the Clarendon, Loup Fork, and Santa Fe marls, referred to the 

 Pliocene by previous writers. The paper was discussed by Dr. Matthew, 

 who dissented 'from the changes proposed. 



RELATIOXSHIP OF THE GREAT BAslX AXD GREAT PLAINS FAUX A 



BY EDWARD L. TROXELL 



(Abstract) 



The study of the vertebrate fossils in the Marsh Collection from these two 

 widely separated regions suggests a new arrangement of the stratigraphic 

 column, which is here presented, with the conclusion that the Arikaree is not 

 Lower Miocene, but Upper Oligocene. and that the dividing line between the 

 latter and the true Miocene should be above the Rosebud and Harrison forma- 

 tions. 



(Treat Plains Great Basin 



[ t, ,1 i Mascall. 



Lower Miocene J Pawnee Creek. _, , , . , 



I Columbia lavas. 



Unconformity. Uppei Oligocene 



Upper Oligocene Rosebud-Harrison. Upper John Day. 



(Arikaree) Monroe Creek-Gering Middle John Day. 



Not only is there a great similarity between the John Day formation of 

 Oregon and the Arikaree of Dakota and Nebraska, but these strata seem to be 

 synchronous with the Oligocene of the Old World. 



The following important groups of animals reach extinction in the Arikaree : 

 The hypothetical Upper Oligocene entelodont (Dinohyus), Steneofiber, Dkera- 

 therium (Ccenopus, Metacwnopus, Menoceras), Eporeodon-Mesoreodon, Leptau- 

 chenia, Mcsohippus-Miohippus, Protoceras-Syndyoceras, and Hypisodus, while 

 with the true Miocene we have the introduction of the proboscideans, teleo- 

 eerine rhinoceroses, and horned artiodactyls. 



Tbe meeting was then adjourned. 



The session of the Vertebrate Section was resumed at 9.30 a. m.. Fri- 

 day. December 30, Dr. Sinclair in the chair. The following papers were 

 presented : 



