PETERSON: THE AMERICAN DICERATHERES. 



403 



fessor Loomis. Among Professor Osborn's third phylum of the later Miocene 

 Rhinoceroses^ we may find a representative of this phylum. 



In Europe Diceratherium pleuroceros (Duvernoy) is the most completely 

 preserved type from the Aquitanian. Its geological horizon apparently approxi- 

 mates in age the John Day beds of North America. 



From the cast of this European species (Fig. 1) it is seen that the cranium back 

 of the orbit is very suggestive of D. annectens. The brain-case has similar small 



Fig. 1. Diceratherium pleuroceros (Duvernoy). From a plaster replica in the Carnegie Museum. X 1/6. 



proportions, the supra-orbital ridges converge gently to form a similarly short 

 sagittal crest, though less prominent and more rounded in the European form. 

 The inion is also somewhat higher in the latter. The muzzle is long, though 

 higher, and perhaps having more the proportions of that part in D. niobrarense 

 from the Nebraska Miocene. The basi-cranium in the cast of D. pleuroceros 

 is short and the mastoid process is in touch with the post-glenoid process. Thus 

 the contour of the skull of the European species apparently has combined char- 

 acters of D. annectens from the John Day and of D. niobrarense from the Nebraska 

 Miocene. The dentition of the European form is too much worn to allow accurate 

 comparison. By regarding such forms as Protaceratherium^ {'^Diceratherium") 

 minutum (Cuvier) of the Stampian as approximately parallel to Coenopus of the 

 upper and middle Oligocene of North America, it appears that the family may be 

 traced back to nearly the same geologic time in Europe and North America,^ 



'Bull. Amer. Mus., Vol. XX, 190-i, p. 321; Aphelops {IPeraceras) planiceps, p. 322; Aphelops 

 {1 Diceratherium) brachyodus p. 324. 



•■Abel, 0., "Kritische Untersuchungen iiber die paliiogenen Rhinocerotiden Europas," Abh. der K. 

 K. Geologischen Reichsanstalt, Band XX, Heft 3, 1910, p. 10. 



* Osborn, Henry F., "Phylogeny of Rhinoceroses of Europe," Bull. Amer. Mus. Nat. Hist., Vol. 

 XIII, 1900, p. 229-267; "Age of Mammals," p. 90. 



