New specimens of the North American Tertiary didelphid marsupials Herpetotherium fugax Cope and Peradectes elegans Matthew and Granger are described. Herpetotherium is shown to be a valid genus, distinct from Peratherium Aymard and Amphiperatherium Filhol, possessing procumbent lower incisors, the first two of which are enlarged. The associated upper and lower dentition of Herpetotherium shows H. fugax to have been a small didelphine of probable insectivorous habits; its molar function centered on a wearing of the crowns in near-horizontal planes and on orthal prevallid–postvallum shear. In P. elegans, by contrast, the lower incisors are unspecialized; P1 is crowded against a robust canine, the posterior lower premolars are trenchant, the molar trigonids are erect, and the talonids anteroposteriorly shortened. Shear in near vertical planes was a more important dental function than in H. fugax. The dental anatomy of H. fugax and P. elegans corroborates recognition of the tribes Didelphini and Peradectini; the last common ancestor of these clades was probably Late Cretaceous in age.

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