Order †Ornithischia Seeley, 1887 Suborder †Ornithopoda Marsh, 1871 see Infraorder Ornithopoda Marsh, 1881 Family †Pachycephalosauridae Sternberg, 1945 Genus †Homalocephale Maryañska & Osmólska, 1974 Genus †Pachycephalosaurus Brown & Schlaikjer, 1943 Genus †Prenocephale Maryañska & Osmólska, 1974 Genus †Stegoceras Lambe, 1902