Holotype of Didymoceras nebrascense (USNM 469). Original unmodified images provided by courtesy of the Smithsonian NMNH and adapted and presented here under a Creative Commons BY-NC 4.0 license (NMNH GUID http://n2t.net/ark:/65665/3fac1e44b-6640-4962-9450-7f3c8fce2b8d). Didymoceras nebracense from the Pierre Shale of Custer County, South Dakota (YPM 006093).