The saurischian, or "lizard-hipped" dinosaurs, like all other tetrapods, had pelves (hips) composed of three elements: the ilium, ischium, and pubis. What distinguishes saurischians (among other major characteristics; including a grasping hand, asymmetrical fingers, and a long, mobile neck) is the pubis that points downward and forward at an angle to the ischium.
What makes an ornithischian dinosaur? All terrestrial animals and even marine animals derived from terrestrial stocks have hip girdles, or pelvises, and all hip girdles are composed of three bones: the ilium, ischium, and pubis. All ornithischians are united by a pubis pointing backward, running parallel with the ischium. The name "Ornithischia" means "bird-hipped," and birds also have pelvises in which the pubis points backwards. However, birds are more closely related to the Saurischia, or "lizard-hipped" dinosaurs, than to the ornithischian dinosaurs...