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pachycephalosaur
(păkˌĭ-sĕfˈə-lə-sôrˌ)
noun Any of various medium-sized herbivorous dinosaurs of the genus Pachycephalosaurus of the late Cretaceous period of North America, having a beaked mouth, rows of bumps on the head, and a domed skull up to ten inches (25.4 cm) thick.