410 MEMOIRS OF THE CARNEGIE MUSEUM. 



made should rather be regarded as in the main due to the varying extent to which 

 speciahzation has operated in the individual. The teeth, especially of the smaller 

 American species of the Diceratheres of the Nebraskan Miocene, may be said to be 

 in a stage of rather rapid and progressive change. It is hardly probable that 

 we shall be able to perfect any satisfactory adaptive radiation of forms, such as 

 has recently been suggested,^ from the study of this material. In paleontology 

 we are debarred from the finer subdivisions used in recent zoology. We have to 

 content ourselves with characters which stand out more prominently and which may 

 be used not only to clearly determine species, but to give aid in the question of cor- 

 relations of faunae and demarcations in geology. From the study of the collection 

 above tabulated, we are forced to regard the variations shown as being individual, 

 sexual, juvenile, and pathological. 



1. Rhinoceros (?Diceratherium) pacificus Leidy," incertce sedis. 



Type. — Upper molar, left side. United States National Mu- 

 seum. 



Horizon. — ? Miocene. 



Locality. — "Alkali Flat" John Day region, Oregon. 

 Fig. 5. Di- Pciratype. — A mutilated fragment of the upper jaw of the right 



ceratherium pa. gide, with portions of the fangs of the true molars and an inferior 

 cificum Leidy, ^^^^^^. ^^^^^ 



.c^ T j^ Horizon. — ? Miocene. 



After Leidy. 



Locality. — Bridge Creek, John Day region, Oregon. 



As indicated in Leidy 's original description this material from "Alkali Flat" 

 in the John Day region, Oregon, was provisionally referred to Coenopus (R.) occi- 

 dentalis. Receiving more material from the same general region Leidj^ again 

 restudied the "Alkali Flat" specimens and finally placed them, together with the 

 material from Bridge Creek, under his species R. pacificus. 



This type like that of the John Day material referred to as R. hesperius we 

 now find to be inadequate, or of very doubtful generic value. Leidy was appar- 

 ently not clear as to the true association of these different fragments and teeth. 

 On page 222 (I.e.) he states that the second molar described, "may be a true molar 

 of the preceding species" [R. hesperius] described in his report. 



I am unable to agTee with Dr. Loomis in accepting this species as valid and 

 am obliged, as the result of the study I have made, to regard this type as incertce 



' Loomis, F. B., I.e., p. 53. 



'» Proc. Acad. Nat. Sci., Philadelphia, 1870, p. 112; 1871, p. 248; U.S.G.S. Terr., Vol. I, 1873, p. 221. 

 Plates II, VII, Figs. 6-7, 24-25; Ainer. .lour. Sci., Vol. XXVI, 1908, p. 55-56, Fig. 6. 



1 



