248 Bulletin American Museum of Natural History. [Vol. XIII, 



long and straight lower jaw of this race. (A maxilla, mistak- 

 enly catalogued A. incisiviim, Georgensgemiind, belongs to the 

 Ceratorhine or C. sansaniensis race.) Vienna : A large maxilla, 

 containing pm i-m 3 (without label) has all the distinctions of 

 the Aceratherine race. 



6. Lower Pliocene Stage. 

 Eppelshcim, Maragha. 



Relations of A. incisivum to Elasmotherium. 



Aceratherium incisivum Kaup. — Darmstadt : In cranial 

 characters this classic species is less dolichocephalic. In dental 

 characters it follows closely upon its predecessors (Fig. 8 E) ; in 

 fact, most writers, beginning with Kaup, have not hesitated to unite 

 the A. tetradactylufn with this animal. The cranial characters, how- 

 ever, are much more progressive, the nasals are shorter and more 

 upturned, the frontals are thickened and bore a rudimentary horn 

 in the males at least. The latter character (Osborn, 99, p. 162) is 

 very significant. One can imagine that this phylum, having failed 

 in the development of horns upon the mechanically weak nasals (as 

 indicated in the Lyons specimen), began to evolve frontal horns. 



There is no question that 



;■, Frontal. 



Nasal. 



the frontals are not only 

 thickened to support a horn 

 (they are very thin in the 

 contemporary T.drae/iypus), 

 but that they show a well 

 marked rugosity with the 

 characteristic converging 

 depressions of nutrient ar- 



Fig. 10. AceratheriujH incisivum. Type: i-pripq/Ficr rn^ Tt is this 



Darmstadt. Rudimentary front.il rugosity, with teries\^rig. lo;. ±L lb Llllb 

 lines of convergent nutrient arteries. character which led the 



writer to advance the idea that this animal is an ancestor of 

 Elasmotherium, an hyi)othesis which depends upon the future 

 discovery of intermediate forms. It may be observed here, more- 

 over, that Elasmotherium has long, narrow, smooth nasals of a type 

 found only in the Aceratheriinae and that there is theoretically no 



