Dinosaurs belonging to this order were particularly abundant during the Jurassic and are characterized by hip bones that are similar to those of modern lizards. These dinosaurs were first discovered in rocks of Triassic age and did not become extinct until the end of the Cretaceous. The lizard-hipped reptiles are divided into two rather specialized groups of dinosaurs: the _theropods_ (carnivorous bipedal dinosaurs that varied greatly in size) and the _sauropods_ (herbivorous, quadrupedal, semi-aquatic, usually gigantic dinosaurs). SUBORDER THEROPODA.— This type of saurischian dinosaur walked on bird-like hind limbs, and they were exclusively meat-eating forms, such as _Allosaurus_ (Pl. 44) of Jurassic age. Some theropods were exceptionally large and were undoubtedly vicious beasts of prey. This assumption is borne out by such anatomical features as the small front limbs with long sharp claws for holding and tearing flesh, and the large strong jaws which were armed with numerous, sharp, dagger-like teeth. The largest of all known theropods was _Tyrannosaurus rex_ which, when standing on his hind limbs, was almost 20 feet tall. Some individuals were as much as 50 feet long, and _Tyrannosaurus_ is believed to have been among the most vicious animals to ever inhabit our earth. A cast of the skull of one of these great beasts is on display in the Texas Memorial Museum at Austin, and a _Tyrannosaurus_ tooth has been found in the Big Bend National Park in Trans-Pecos Texas. SUBORDER SAUROPODA.— The sauropods were the largest of all dinosaurs, and some attained a length of 85 feet and probably weighed 40 to 50 tons (_Brontosaurus_, Pl. 44). They were primarily herbivorous dinosaurs which had become adapted to an aquatic or semi-aquatic type of existence and probably inhabited lakes, rivers, and swamps. The tracks of sauropod dinosaurs have been collected from Lower Cretaceous rocks in central Texas (Pl. 4) and Upper Cretaceous beds in Big Bend National Park in Trans-Pecos Texas. Order Ornithischia.— The ornithischian, or bird-hipped dinosaurs, were herbivorous reptiles which were quite varied in form and size and appear to have been more highly developed than the saurischians. This order includes the duck-billed dinosaurs (ornithopods), the plate-bearing dinosaurs (stegosaurs), the armored dinosaurs (ankylosaurs), and the horned dinosaurs (ceratopsians). Ornithischian tracks are known from Cretaceous rocks in central and Trans-Pecos Texas. SUBORDER ORNITHOPODA.— These unusual dinosaurs were predominantly bipedal, semi-aquatic, and some (like the duck-billed dinosaurs) were highly specialized (_Trachodon_, Pl. 45). SUBORDER STEGOSAURIA.—