The Judithian (Late Cretaceous) stagodontid marsupial Eodelphis Matthew is known from two closely related species, E. cutleri and E. browni, which differ in size and proportions of P3 and robustness of the molars. P1 is shown to be two-rooted in both species and not only in E. cutleri, as previously believed. Known upper molars of Eodelphis lack a stylar cusp D, indicating that neither E. cutleri nor E. browni can be a part of the direct (species) ancestry of the later Cretaceous stagodontid Didelphodon. Evolution of the Stagodontidae probably included derivation of an E. cutleri-like radicle having robust premolars and molars from E. browni-like ancestors, present in North America by Aquilan time; development by the Edmontonian of specialized crushing premolars in primitive species of Didelphodon, descended from ancestors that were similar to E. cutleri in size but had a prominent stylar cusp D on the upper molars; and subsequent increase in size leading to Lancian D. vorax and D. padanicus. Contrary to recent speculation, the Stagodontidae have no special relationship to the Lower Cretaceous Pappotheriidae, which are Eutheria.