Heteromorph ammonites of the families Nostoceratidae Hyatt, 1894, and Diplomoceratidae Spath, 1976, are common to abundant in sediments deposited in the western and central parts of the U.S. Western Interior Seaway during the early late Campanian. The indices of the successive zones of Didymoceras nebrascense (Meek and Hayden, 1856a) (oldest). Didymoceras stevensoni (Whitfield, 1877), Exiteloceras jenneyi camacki, n. subsp., Exiteloceras jenneyi jenneyi (Whitfield, 1877), and Didymoceras cheyennense (Meek and Hayden, 1856a) are revised as are Nostoceras monotuberculatum Kennedy and Cobban, 1993a (D. nebrascense and D. stevensoni zones), Oxybeloceras crassum (Whitfield, 1877) (D. stevensoni and E. jenneyi zones) and Spiroxybeloceras meekanum (Whitfield, 1877) (D. cheyennense zone). Solenoceras elegans n. sp. (D. stevensoni and E. jenneyi zones), Solenoceras bearpawense, n. sp. (D. nebrascense zone), and Solenoceras larimerense, n. sp. (E. jenneyi zone) are also described.
