Hylaeosaur \Hy"l[ae]*o*saur`\, Hylaeosaurus \Hy`l[ae]*o*sau"rus\, n. [NL. hylaeosaurus, fr. Gr. ? belonging to a forest (fr. ? wood) + ? a lizard.] (Paleon.) A large Wealden dinosaur from the Tilgate Forest, England. It was about twenty feet long, protected by bony plates in the skin, and armed with spines.