WordNet (r) 3.0 (2006):

Ornithischia
    n 1: extinct terrestrial reptiles having bird-like pelvises:
         armored dinosaurs (thyreophorans); boneheaded and horned
         dinosaurs (marginocephalians); duck-billed dinosaurs
         (euronithopods) [syn: Ornithischia, order Ornithischia]