﻿lxx PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. lxxii, 



molars in complexity ; while the successive representatives of the 

 family showed a progressive increase in bodily bulk. These 

 changes took place almost simultaneously both in the Old World 

 and in North America, though in the latter region the whole 

 family died out in early Pliocene times as soon as a rudimentary 

 nasal horn was beginning to appear. After the earliest stage, 

 however, the European series at least can readily be distinguished 

 from the American series by the premolar teeth, and there must 

 have been independent evolution in the two regions. According 

 to Prof. 0. Abel, the oldest known recognizable member of the 

 family is the small Prohyracodon from the Middle Eocene of 

 Transylvania ; and both this and the later Epiaceratheriam, 

 from the Lower Oligocene of Northern Italy, agree with the 

 earliest North American Rhinoceroses in having the foremost 

 premolar more nearly like a molar than is the hindmost premolar. 

 They thus show the first step in the complication of the premolars 

 from front to back, which, according to Prof. H. F. Osborn, is 

 the direction of this complication during geological time in all 

 the American Rhinoceroses. On the same account, they differ 

 from all the known Rhinoceroses which followed them in Europe : 

 for in these the complication of the premolars always began 

 with the hindmost, and proceeded forwards. Not only can two 

 distinct progressive groups be thus recognized in two distant 

 regions, but more than one line of development can also be traced 

 within each. The fossils, indeed, seem to justify the conclusion 

 that the Rhinoceroses arose from a common stock, but evolved in 

 several different ways and at varying rates towards the same goal 

 of specialization. 



It is curious that extreme specialization in the family was 

 reached only in the Old World, and still more curious that the 

 large-horned woolly rhinoceros (Ph. antiguitatis), which lived in 

 the Arctic Regions with the mammoth, never accompanied that 

 animal to North America. Equally interesting is the exceptional 

 case of high specialization in the Old- World Pleistocene Plasmo- 

 therpum, in which the extreme deepening, enlargement, and 

 complication of the molar and premolar teeth seem to have been 

 correlated with the reduced development of the epidermal horn. 

 The large bony boss on the frontals of this animal was probably 

 covered only by a very thin capping of horn. At least, it is 

 difficult to suggest any other interpretation for such a horn, which 

 was found isolated a few years ago by the Prussian Geological 



