BIG BADLANDS OF SOUTH DAKOTA. 77 



lists published as far back as 1909. Hyracodon major, if properly 

 referable to the genus, is specifically indeterminate and should be 

 abandoned. The type consists of some skeletal elements in the 

 Princeton collection, No. loooi, of unknown stratigraphic position 

 other than that they are from the White River Oligocene of Dakota, 

 and may be a little larger than the corresponding skeletal parts of the 

 largest individuals of certain of the species here recognized. It is 

 possible that still larger individuals of these existed than have so far 

 appeared in the collections, but the diagnostic characters for their 

 specific determination must depend, in any event, on the upper pre- 

 molars and not on the skeleton. 



This completes a survey of the nomenclature and synonymy of 

 the genus to date. 



Range in Time. 



The following table shows the range of species in time and in- 

 cludes several specimens from the American Museum collection, not 

 referred to in the text, kindly placed at my disposal by Professor 

 Osborn and Dr. Matthew. It will be seen at once that each one of 

 the four species recognized is not confined to a definite zone, but 

 overlaps widely, in turn, on the time limits of the next and more 

 advanced type. If Mr. Troxell is correct in placing H. leidyanns as 

 low as the Titanotherium beds, it may well be that the divergence of 

 all four species took place there, or even earlier, before the appearance 

 of Hyracodon in the North American non-marine Oligocene of the 

 plains region, to which it is, so far, narrowly confined and of which 

 it is an excellent index fossil. For zonal purposes the species are of 

 little value, as they range through a considerable thickness of beds, 

 and the same is also true of the size variants. 



In this table the heavy black lines show the position of the lower 

 and upper zones of rusty nodules in the Lower Oreodon beds, both 

 of which are properly included in the Mesohippus bairdii-Oreodon 

 culhertsoni zone, the upper and lower limits of which have not yet 

 been established. The specimens discussed in the table are from the 

 following localities : 



Hyracodon arcidcns Cope. 



8807 Am. Mus. Nat. Hist. Oreodon beds, probably upper 

 according to Dr. Matthew, Logan Co., Colorado. 



