38 AMERICAN MESOZOIC MAMMALIA 



tions since Owen have agreed in considering the symmetrodonts and triconodonts as 

 very closely related. The taxonomic history of the group need not be repeated here. 



Provided that the essential unity of the group and its fundamental differences 

 from the Triconodonta and somewhat less fundamental but still very important differ- 

 ences from the Pantotheria are recognized, it is of relatively little importance whether 

 it is given ordinal rank or less. Subordinal position under the Pantotheria might be 

 equally justified, although the supposed transitional steps to the true pantotheres do 

 not exist and any actual link between the two groups is as yet unknown. 



Like the triconodonts, the symmetrodonts have recently been treated in detail by 

 the author (1925c, and 1928B), so that formal definitions with such slight changes as 

 further study has indicated will here suffice. The following classification is proposed : 



CLASS MAMMALIA Linnaeus 1758 



SUBCLASS THERIA Parker & Haswell 



ORDER SYMMETRODONTA Simpson 1925 

 Family Spalacotheriidae Marsh 1887 

 Sfalacotherium Owen 1854 

 Peralestes Owen 1871 

 Tinodon Marsh 1879 

 Family Amphidontidae Simpson 1925 

 Amfhidon Simpson 1925 

 Incertae sedis: 



Eurylambda Simpson, new genus. 



Both families and the last three genera occur in the American Morrison forma- 

 tion. 



SPALACOTHERIIDAE Marsh 1887 



Definition. — Lower molars functionally tricuspid. Antero- and posterointernal 

 cusps distinct and well developed, springing from base of crown. Internal cingulum 

 continuous, rising to form small anterior and posterior cingulum cusps. 



Type. — Sfalacotherium Owen. 



Distribution. — Upper Jurassic of England and the United States. 



Tinodon Marsh 1879 



1879. Tinodon, Marsh, Amer. Jour. Set. (3) XVIII, 216. 

 1887. Menacodon, Marsh, Amer. Jour. Set. (3) XXXIII, 340. 



Definition. — Dental formula I, Ci P3 M4. Molars longer than high, no external 

 cingulum. Premolars symmetrical, not recurved. Coronoid arising at an angle of about 

 90° to the alveolar border. 



Type. — Tinodon bellus Marsh 



Distribution. — Upper Jurassic, Wyoming. 



