94 M. R. Thorpe — John Day Eporeodons 



John Day lias no designation, as it has so far yielded 

 extremely scanty and ill-preserved vertebrate remains. 

 The middle is termed the Diceratherium zone (Wortman), 

 and the upper, the Promerycochoerus zone (formerly 

 Merycochoerus). It is the writer's opinion that the term 

 Diceratherinm zone should be replaced by Eporeodon 

 zone, for the following reasons : 



(1) Diceratherium is known to occnr in both the middle 

 and upper series in considerable abundance; (2) it is 

 quite difficult to identify Diceratherium in the field, omng 

 to the general lack of skull and tooth material, and due to 

 the fragmentary condition of the skeletal elements as 

 usually found; (3) the Eporeodon material in the Marsh 

 and other collections shows conclusively that representa- 

 tives of this genus are the most common fossil vertebrates 

 in the middle John Day. As a result of studies carried 

 on at the University of California, the fact is brought out 

 that 



"The middle John Day is characterized by abundant remains 

 of Eporeodon, especially of the two smaller species occidentalis 

 and pacificus, and by a great number of rodents, which have 

 been obtained principally at two horizons, one at about the 

 middle and the other at the top of the Diceratherium beds. Both 

 horizons contain practically the same fauna. Next to Eporeodon 

 in abundance the most common form is a Hypertragidus, frag- 

 mentary specimens of which occur at almost every exposure. 

 Rhinoceros material is fairly abundant but usually fragmentary 

 and possibly represents other genera in addition to Dicerather- 

 ium.'' (Merriam and Sinclair 1907, p. 190.) 



(4) The Oreodontidae are such a wide-spread family 

 and bear so great a similarity to each other in skeletal and 

 tooth structure that their identification would be much 

 easier to all field men, except expert paleontologists, than 

 would that of Diceratherium, especially when only bone 

 fragments of the latter are present. (5) There would 

 be no confusion in distinction between Eporeodon of the 

 middle and Promerycochoerus of the upper series, as the 

 two genera have certain invariable distinctions clearly 

 defined. 



The study of specimens of this genus from the North 

 Fork of the John Day River is unsatisfactory at best, due 

 to inadequate geologic data concerning that area. It 

 seems advisable at present to assume that these beds are 

 of later origin than the typical John Day formation, and 



