A thickness of from 1000 to 2000 feet of strata in western Wyoming, southeastern Idaho, and southwestern Montana is shown to belong to the Otoceras, and probably to the Genodiscus, zones of the Scythian stage of the Lower Triassic. These strata, herein classified as the Dinwoody and Woodside formations, exhibit marked overlap relations on the underlying Phosphoria formation, indicative of a marked hiatus corresponding to the Permo-Triassic boundary. Unlike current usage in southeastern Idaho the term Woodside is herein employed for unfossiliferous red beds, which apparently grade northeastward into a nonred fossiliferous section similar to the underlying Dinwoody formation. Further work remains to be done in the areas in which the red-beds facies is lacking. The marine invertebrate fauna of the Dinwoody has a marked affinity with the Otoceras fauna from east Greenland, Seis (lower Werfen) fauna of the Alps, and early Triassic faunas from eastern Siberia.