Several biologically significant domes of pachycephalosaurid dinosaurs are described and figured. One unusual specimen from the Oldman Formation (Upper Cretaceous) of Alberta is placed into a new genus and another specimen from the same formation is assigned to a new species of Stegoceras. Domes referable to Stegoceras sp. (Judith River and Hell Creek Formations) are the first conclusive evidence of the presence of this genus in the United States. A large dome from the Oldman Formation of Alberta is referred to Pachycephalosaurus; this specimen is the oldest described to date and is the first record of this genus in Canada.Endocranial casts of Yaverlandia bitholus and Stegoceras validus are described along with a discussion of endocranial trends in pachycephalosaurids. The separation between the cerebrum and cerebellum found in Yaverlandia, and typical of ornithopods in general, is lacking in Stegoceras and Pachycephalosaurus. The loss of this separation may be the result of head butting.The family Pachycephalosauridae possesses sufficient ornithopod characters to justify their retention in that suborder. It does, however, represent an aberrant side branch of ornithopod evolution, which can best be visualized taxonomically by placing this family into a separate infraorder within the Ornithopoda.