Troxell — Hyrachyus and its Subgroups. 39 



may be combined into a harmonious genus with certain 



well defined features and yet with certain individuali- 

 ties which indicate a rather wide variation. 



As to what constitutes the type of the genus, there 

 has been much misunderstanding. Through a passive 

 tolerance, authors have come to accept H. agrarius 

 Leidy as the type, following Leidy, who himself chose 

 to eliminate H. agrestis, a species founded upon the lower 

 jaw of a young animal with milk teeth. 



Although Leidy stated specifically in his first descrip- 

 tion of the genus (1871 A, p. 357) that Hyraehgus "an 

 extinct genus, allied to Hyraeodon, is founded on a frag- 

 ment of a lower jaw of a young animal/' yet in a later 

 paper (1873, p. 60) he figures this jaw but says that he 

 regards it as the same species as H. agrarhis, and sub- 

 stitutes the latter as the genoholotype. This has been 

 followed in the literature with only one or two excep- 

 tions. It is of no great moment which of these species 

 represents the genus, but the rules of nomenclature 

 demand that we adhere strictly to the original type in 

 spite of the desires of later writers, including the nomen- 

 clator himself, and so H. agrarius must give place to 

 H. agrestis. 



This species is reinstated as the genoholotype with 

 greater confidence because of the following statement 

 from Doctor 0. P. Hay, who has recently looked into 

 this question and says in a letter dated June 18, 1921, 

 "It is evident that Leidy meant to base his genus on 

 H. agrestis." 



Neither the holotype of H. agrestis nor that of H. 

 agrarius is specifically determinable. The first is based 

 on a lower jaw fragment containing the first and fourth 

 deciduous premolars, the roots of the intervening two, 

 and also the first molar of the permanent series ; the 

 second type is founded upon a lower jaw without the 

 crowns of any teeth. 



Definition of the Genus. — Hyraeliyus Leidy may be 

 distinguished as having smooth nasals without horn 

 rugosities ; diastemata separating the canines and 

 premolars; canines moderately long, pointed, and only 

 slightly flattened; premolars with cross crests not 

 parallel but forming a loop, the metaloph encircled by 

 the protoloph; molars with strong parastyles consti- 



