A long-standing taxonomic error is corrected by recognizing that the previously enigmatic early Paleocene (Torrejonian) mammal Microclaenodon assurgens (Cope, 1884) from western North America is a subjective junior synonym of Oxyclaenus cuspidatus (Cope, 1884). Carcinodon simplex (Cope, 1884) (= C. filholianus) is suggested as a new combination. Specimens previously incorrectly referred to O. cuspidatus are tentatively referred to the genus Carcinodon as a new species, “C.” olearyi. A phylogenetic analysis that reconstructed the relationships of Oxyclaenus, “Carcinodon,” selected “triisodontids,” selected basal ungulates, and basal mesonychians incorporates 30 taxa and 66 dental characters and results in up to 210 MPTs, each 180 steps long, with a C.I. of 0.51 and a H.I. of 0.49. O. cuspidatus is resolved as the sister species of Andrewsarchus mongoliensis. The Andrewsarchus Oxyclaenus clade is the sister lineage of Mesonychia. Outside of this grouping, “triisodontids” form a polytomy, which is followed by a clade composed of “C.” olearyi n. sp. and “C.” antiquus.