68 SINCLAIR— HYRACODONS FROM THE 



with that shown in a specimen from the Titanotherium beds at Bone 

 Coulee, Cypress Hills, Saskatchewan, and designated by Lambe^ 

 Hyracodon priscidens. Cope's^ brief characterization of Hyracodon 

 arcidens makes it evident that he had before him a specimen showing 

 the same features as does our No. 12518 (Fig. i) from the zone of 

 rusty nodules, forty feet, more or less, above the base of the Oreodon 

 beds in Indian Creek, Pennington County, South Dakota. Similarly, 

 Troxell's Hyracodon arcidens mimus^ from the Oligocene at Dead- 

 wood, South Dakota, and his Hyracodon seienidens^ from the Middle 

 Oligocene of Colorado are size variants within the same structural 

 range. Cope's Hyracodon arcidens must take priority. The holo- 

 type of this species seems to be misplaced, as the specimen so recog- 

 nized by the American Museum (No. 6309, Am. Mus. Nat. Hist.) 

 and figured as the holotype in the Cope plates'' does not agree with 

 Cope's description of the specimen on which his species was based. 

 The so-called holotype, from the Cedar Creek beds (Oreodon zone) 

 of Logan County, Colorado, is an old individual with the crown pat- 

 terns of the premolars obliterated by wear and is practically indeter- 

 minate specifically. Hyracodon priscidens Lambe, H. arcidens mimus 

 Troxell and H. selenidens Troxell take precedence in the order given. 

 The various specimens of what we may now speak of as Hyra- 

 codon arcidens, when arranged in order of size, are seen to intergrade 

 by small increments as follows : 



Cope's type premolar series, length 72 mm J 



Princeton Geol. Mus. No. 10816 premolar series, length 71.5 



Princeton Geol. Mus. No. 12518 premolar series, length 67 



H. arcidens mimus, Yale P. M. No. 11 174 ... .premolar series, length 67^ 



Lambe's type of H. priscidens premolar series, length 57^ 



H. selenidens, Yale P. M. No. 11 173 premolar series, length 57' 



2 L. M. Lambe, Trans. Royal Society of Canada, second series, 1905-6, 

 Vol. XL, Section IV., pp. 37-42, Plate I, issued August, 1905. 



3 E. D. Cope, Palseontological Bulletin No. 15, p. 2, issued August 29, 



1873. 



4 E. L. Troxell, loc. cit., p. 36. 



5 Ibid., p. 37. 



^ Hitherto Unpublished Plates of Tertiary Mammalia and Permian Ver- 

 tebrata. U. S. G. S. and Am. Mus. Nat. Hist., 1915, Plate CII. I am told 

 that this specimen was marked as the type on Cope's label which accompanied 

 it. 



