In this paper several new significant specimens of pachycephalosaurid dinosaurs from Upper Cretaceous strata of Canada and the United States are described and figured. The domes of an unusual pachycephalosaurid from the Judith River Formation (Campanian) of Alberta show a slightly thickened frontal with an ornamentation of bony tubercles; they are referred to a new genus. A new genus of Pachycephalosauridae is described from the Hell Creek Formation (Maestrichtian) of Montana; it has horn-like projections on the squamosal. Sutural fusion between the quadrate and squamosal of Pachycephalosaurus is reported; this is an unusual condition among the Ornithischia. The biological significance of certain cranial features is considered.