Genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Dracorex in MT, SD and WY and all are found in the Hell Creek and Lance Formations although my specimens are just from the HC. I've identified these domes based on my, best guess, using accepted convention, however, in 2007 Horner presented a theory which proposed that Dracorex hogwartsia and Stygimoloch spinifer are growth stages of Pachycephalosaurus wyomingensis and represent an ontogenetic series of P. wyomingensis with Dr