Plastomenidae is a speciose clade of soft-shelled turtles (Trionychidae) known from Campanian to Eocene deposits throughout western North America. We here describe two large skulls from the Upper Cretaceous (Maastrichtian) Hell Creek Formation of Carter County, Montana, that document the adult morphology of the plastomenid Gilmoremys lancensis. Whereas juveniles of this species, as previously documented by five subadult skulls, have narrow skulls, a narrow processus trochlearis oticum, a deep and narrow median palatal groove, low accessory ridges, and a secondary palate fully formed by the maxilla, skeletally mature individuals have notably broad skulls, a broad processus trochlearis oticum, a shallow but broad median palatal groove, high accessory ridges, and a substantial contribution of the vomer to the secondary palate. An expanded phylogenetic analysis reveals that the Campanian Aspideretoides foveatus and the Paleocene Aspideretoides superstes, nov. comb., are situated within Plastomenidae as sister to all previously identified plastomenid turtles, despite their general resemblance to trionychine soft-shelled turtles. The name Aspideretoides should therefore not be used as a taxonomic wastebasket for fossil trionychids with unclear phylogenetic relationships.