Ever wonder what that prong of bone in the orbits of ornithischian dinosaurs is called? I've often wondered that myself, and I accidentally came across the answer this morning while reading through Currie & Carpenter's Acrocanthosaurus description.
It's the palpebral bone! And it evolved independantly among many crocodilians and ornithischians. The feature seems primitive to the Ornithischia, although many groups subsequently lost it. Others enlarged it, among them "hypsilophodonts" (above) and basal ceratopsians. In his Predatory Dinosaurs of the World, Greg Paul suggests that the palpebral would have given ornithischians "eagle eyes." The bones would have certainly been covered with skin, and would have given the eyes of many ornithischians a certain "evil" glare. What purpose the palpebral bones served is not known (and I've never really seen it discussed in the literature). And why did some groups need it while others lost it completely?