Twenty-seven inoceramid species and subspecies of the genera Inoceramus J. Sowerby, 1814, Mytiloides Brongniart, 1822, Cremnoceramus Cox, 1969, and Tethyoceramus Sornay, 1980 are described from the Upper Turonian and Lower Coniacian of the Western Interior of the United States. One Inoceramus species, I. dakotensis, and two Mytiloides species, M. ratonensis, and M. bellefourchensis, are described as new, and four forms are left in open nomenclature. Four widely used North American species, I. dimidius White, 1874, T. perplexus Whitfield, 1877, C. deformis erectus (Meek, 1877), and C. deformis deformis (Meek, 1871), are thoroughly discussed for the first time and illustrated. The inoceramids allow a subdivision of the Upper Turonian and Lower Coniacian into eight zones, and their precise correlation with the European succession and the standard substage subdivision of the Turonian and Coniacian, as well as with the ammonite zonation used in the US Western Interior. The base of the Upper Turonian, corresponding to the base of the European I. costellatus Zone, corresponds to the base of the I. perplexus Zone, and to the base of the Scaphites whitfieldi arrunonite Zone. The Upper Turonian is divided into zones of Inoceramus perplexus, I. dakotensis, Mytiloides incertus, M. scupini, and Cremnoceramus waltersdoifensis waltersdoifensis. The Lower Coniacian has the zones of Cremnoceramus deformis erectus, C. deformis dobrogensis, and C. crassus crassus. In the upper part of the erectus Zone and in the dobrogensis Zone, the Tethyoceramus wandereri and Cremnoceramus crassus inconstans zones may be distinguished. Cremnoceramus deformis erectus (Meek) is the basal member of the deformis lineage and the proper name for forms referred to C. rotundatus (sensu Troger 1967 non Fiege 1930), and, accordingly, is the basal boundary marker of the Coniacian. So defined, the first Forresteria have already appeared in the topmost Turonian. The Turonian/Coniacian boundary is marked by a series of short-lived events better recognized in Europe (Didymotis I Event, waltersdoifensis Event, herbichi Event, erectus I Event).