E, L. Troxell — New Species of Hyracodon. 35 



Discussion of Known Species. 



Hyracodon nehrascensis (Leidy). 



"A species founded upon a great portion of the face, containing 

 all the superior molar teeth ; an inferior maxilla with six molars ; 

 and three superior, apparently deciduous molars. It is about the 

 same size as the R. miniitus of Cuvier. 

 "Length of line of seven supe- 

 rior molars 4 7/10 inches [119.4 mm.] 

 Length of line of six inferior 



molars 4 2/10 inches [106.7 mm.] 



Breadth of jaws from the first 

 superior true molar teeth of 

 one side to the other 3 8/10 inches [ 96.7 mm.]."^^ 



It is evident from this and subsequent descriptions that 

 Leidy did not limit himself to one single species, but 

 included specimens with varied features. 



Hyracodon arcidens Cope. 



The holotype is primarily based on a maxillary with the 

 premolars and M.^ of a very young animal. Cope says r 



' ' The species is about the size of the H. nehrascensis, and differs 

 in the form of the inner lobes of the molars and of the first premo- 

 lar. All the molars have the outer longitudinal and inner 

 transverse crests, the posterior short, the anterior much curved 

 backward round it, and thus forming the inner boundary of the 

 tooth-wall." 



This is apparently the first true specifiic description we 

 have of a hyracodont; it is obviously similar to certain 

 phases of H. nehrascensis — it could hardly be otherwise 

 — but applies to that distinctive group, moderate in size, 

 which have the anterior crest much curved backward. 



Hyracodon major Scott and Osborn. 



The type of this species is a fairly complete skeleton in 

 the Princeton JViuseum. The species description^ is based 

 on a fore foot and therefore can not be compared to the 

 new species described later in this paper ; unfortunately 

 it does not give any tooth characters and so we know 

 little more than the proportional size of the specimen. 



^Joseph Leidy, Proc. Acad. Nat. Sci. Phila., 5, 121, 1850. 



' E. D. Cope, Pal. Bull. No. 15, 2, 1873. 



•' W. B. Scott and H. F. Osborn, Bull. Mus. Comp. Zool., vol. 13, 170, 1887 



