Cope.) Olb [ Jan . i 8j 



formations fails to reveal any characters distinguishing them as more than 

 one species. In fact the variation in various respects is greater among the 

 individuals of the John Day epoch, than between those of the two epochs. 

 This was by far the most abundant mammal of the John Day epoch 

 while it appears to have been rare during that of the White River. 



Specimens differ in the size of the preorbital fossa irrespective of other 

 differences. In some specimens it is wide and profound, including the 

 lachrymal bone ; in others it is less extensive and is shallow, involving but 

 part of the lachrymal. It is never wanting or obscure. For estimation of 

 other characters, I select ten crania, nine from Oregon and one from 

 Dakota, as expressing the greatest range of variation. Of these, three 

 display a peculiarity in the form of the otic bulla. Instead of being con- 

 tracted backwards in front, it is protuberant and full at its inferior anterior 

 part. Five other crania, agreeing with these three in other respects, 

 possess the normal form of bulla. In one cranium, which is rather more 

 robust than the others, the infraorbital foramen is a little posterior to its 

 usual position, being above the anterior part of the fourth premolar. 

 This tooth is also distinctly smaller than in other specimens of otherwise 

 similar dimensions. The majority of specimens range nearly alike in 

 dimensions, but there are forms distinctly larger and smaller, which may 

 represent distinct species. This question can be better decided when the 

 skeletons are known. I give three sub-species which are defined as fol- 

 lows : 



Length of cranium M. .197; of molar series M. .086; long 

 diameter of base of paroccipital process transverse ; its pos- 

 terior base flat E. j. jacksoni. 



Length of cranium M. .219 ; of molar series M. .091 ; paroccipi- 

 tal process as above E. j. pacificus. 



Length of cranium, M. .235; of molar series, M. .099; paroc- 

 cipital process strongly compressed, its posterior base an- 



gulate on the middle line . . . , E. j. leptacanthus. 



The above measurements of length are made from the occipital condyles 

 to the premaxillary border inclusive. 



The three forms may represent good species. The E. j. jacksoni is of the 

 size of the Oreodon culbertsoni ; the E. j. leptacanthus is larger than the E. 

 major, while the E. j. pacificus is intermediate between the two. 



Eucrotaphus jacksoni jacksoni Leidy. 



The typical specimen of the Oreodon bullatus Leidy agrees so nearly with 

 the original type of Eucrotaphus jacksoni, that I cannot doubt their pert- 

 inence to the same species. There are two specimens in the collection of 

 the Philadelphia Academy, besides the last named, and at least one in the 

 museum at. Princeton. A specimen from the John Day, Oregon, cannot 

 be distinguished from these. It agrees with Marsh's measurements and 

 description of his Oreodon occidentalis, and no doubt represents it. Its 



