390 



H. F. Osbom — Mammalia in North A 



merica. 



typical ancestral formula. The Triconodonts are a newer 

 group, perhaps derived from the Dromotheriidae (incipient Tri- 

 conodonts) of the Trias although these appear to be aberrant ; 

 the typical forms extend from Amphilestes to Triconodont, 



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and exhibit the first stages of development of the inflected 

 Marsupial jaw. The Trituberculates include the Amphi- 

 theriidae and Amblotheriidae with true tuberculo-sectorial 

 lower molars, like those of modern Insectivores ; they alone 

 exhibit the typical angular placental jaw, — no reason can be 

 assigned for calling them Marsupials, excepting the traditional 



