Eutrephoceras dekayi from the Late Cretaceous Pierre Shale Fm. of Pennington County, South Dakota (YPM 219057). Eutrephoceras dekayi from the Late Cretaceous Pierre Shale Fm. of Meade County, South Dakota (FHSMIP 1290). Eutrephoceras dekayi from the Late Cretaceous Pierre Shale Fm. of South Dakota (UNM 4469).