302 



University of California Publications. 



[Geology 



The discovery of the terminal portion of a twisted horn in 

 the form described above as Ilingoceros schizoceras unexpectedly 

 complicates the problem of the relationships of Ilingoceros 

 through the addition of the character of terminal bifurcation to 

 that of spiral twist. While it is by no means certain that the 

 species referred to as I. schizoceras is generically identical with 

 I. alexandrae the evidence suggests that the two are probably 

 of common origin. The presence in I. schizoceras of a terminal 

 bifurcation of the horn as in Merycodus lends some support to 

 the theory that the Merycodontidae may be the ancestors of 

 Ilingoceros. 



In the present state of our knowledge there seem three 

 hypotheses open to account for the presence of the twisted- 

 horned antelopes in the Thousand Creek fauna: (1) They are 

 typical Old World tragelaphines which came into America in 

 late Miocene or early Pliocene time and developed long-crowned 

 molar teeth. (2) They are tragelaphine forms which originated 

 in America from Merycodus-like ancestors at some time during 

 the Miocene, and soon migrated to the Old World, leaving only 

 a few descendants here as late as the Thousand Creek epoch. 

 (3) They are a peculiar twisted-horned division of the Antilo- 

 eapridae originating in America and possibly limited to this 

 continent. 



On the whole, the writer is inclined to think that the evidence 

 favors recognition of a fairly close relationship of Ilingoceros 

 as well as Sphenophalos with the Antilocapridae, and that all of 

 these forms may be derived from some member of the Mery- 

 codus group. With the evidence at hand, one does not seem 

 justified in assuming that the Old World tragelaphines are nec- 

 essarily derived from American Merycodus-like ancestors, 

 though both may have come from the same stock. There seem 

 to be some reasons for thinking that the older tragelaphines of 

 the Old World may have been derived from some form like 

 Palaeomeryx or Dromomeryx rather than from a Merycodus- 

 like type. Dromomeryx of the American Miocene possesses ante- 

 lope-like horns and a dentition which might have developed into 

 the tooth type found in the more primitive of the true antelopes. 



In addition to the evidence suggesting the presence in 



