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ABSTRACTOne of the most enigmatic species of North American mosasaur, Platecarpus planifrons(Upper Coniacian—Lower Santonian), is re-diagnosed based on reference to the holotype, a new and very well-preserved skull (UALVP 24240), and several other less complete specimens. P. planifrons, once considered a nomen vanum, is diagnosed by a number of characters: separation of prefrontal and postorbitofrontal on ventral surface of frontal; dorsal surface of frontal planar with no mid-sagittal keel; width of preorbital region of frontal greater than interorbital width; frontal with paired posteromedian flanges; anterodorsal border of quadrate with distinct notch; stapedial pit narrow and keyhole shaped. The unexpected preservation of paired septomaxillae and the presence of a clear and well-defined vomerpterygoid contact in UALVP 24240 are reported. These new osteological data for P. planifronspermit re-evaluation of the species composition of the genus. Arguments are presented for the synonymy of P. coryphaeuswith P. ictericus, while retaining the latter as distinct from the generic type, P. tympaniticus. Platecarpuscf. P. somenensisis tentatively retained. Thus, four species of Platecarpusfrom the Western Interior Seaway of North America are recognized.