30 AMERICAN MESOZOIC MAMMALIA 



conodonta was first used by Osborn in this memoir ( i888a, p. 251 ) being there applied 

 to a hypothetical group supposed to be ancestral to the triconodonts proper and to 

 certain other mammals {Astheiiodon, Stylacodon'). In a succeeding paper published 

 the same year (iBSBb), Osborn more definitely recognized the presence in the Jurassic 

 of two molar types (in addition to the multituberculate), which he called triconodont 

 and trituberculate. The word "triconodont" hence passed into vernacular usage as a 

 morphological term and, by extension, as a name for any mammal belonging to the 

 group with triconodont molars. Its first use for a definite taxonomic division was ap- 

 parently in Zittel's Handbuch (1893, p. 97) where "Triconodonta Osborn" is made a 

 group under the suborder Polyprotodontia. 



In 1907 (p. 21) Osborn elevated the group to ordinal rank. Only one family is 

 recognized, as before, but under Amphilestinae is only Amfhilestes, while Tricano- 

 don, Triacanthodon, and Priacodon form the subfamily Triconodontinae. The Phasco- 

 lotheriinae are as before, and the Spalacotheriinae (to which Peralestes is added) are 

 considered as incertae sedis. 



Subsequent work has been chiefly based on the literature only and has been 

 largely devoted to the question whether all the genera referred by Osborn to the order 

 belong there. Thus Gregory (1910, p. 173) at first decided that Amfhilestes, Phasco- 

 lotherium, Triconodon, and Priacodon certainly were related and that the other gen- 

 era (our symmetrodonts) probably were correctly referred here. Matthew, however 

 (in Gregory 1922, p. xiii), concluded that Sfalacotherium and its allies had nothing 

 to do with the true triconodonts. 



Finally, the present writer has already proposed a new classification of the order 

 (1925B, p. 357) which differs from Osborn's chiefly in removing the allies of S-palaco- 

 therium and in placing P hascolotherimn in the Amphilestinae rather than in a distinct 

 subfamily. With a generic change to be explained below, this classification has been 

 supported by subsequent work. A complete arrangement in the light of recent studies 

 is here given : 



CLASS MAMMALIA Linnaeus 1758 

 I ncertae sedis : 



ORDER TRICONODONTA Osborn (1888) 

 Family Triconodontidae Marsh 1887 



Subfamily Amphilestinae Osborn 1888 

 Amfhilestes Owen 1859 

 Phascoloiherium Owen 1838 

 PhascolodoH Simpson 1925 

 Afloconodon Simpson 1925 

 Subfamily Triconodontinae Osborn 1907 

 Triconodon Owen 1859 

 Trioracodon Simpson 1928 

 Priacodon Marsh 1887 



The morphology of the American Triconodonts has already been treated (Simp- 

 son 1925B), and it is not proposed to repeat the details here. Formal definitions are 



