ceratopsian: any of several four-footed herbivorous dinosaurs with enormous beaked skulls; of the late Cretaceous in North America and Mongolia
ornithischian: herbivorous dinosaur with a pelvis like that of a bird
ornithischia: extinct terrestrial reptiles having bird-like pelvises: armored dinosaurs (thyreophorans); boneheaded and horned dinosaurs (marginocephalians); duck-billed dinosaurs (euronithopods)