The Dinosaurs originated, so far as we can judge, as lizard-like reptiles with comparatively long limbs, long tails, five toes on each foot, tipped with sharp claws, and with a complete series of sharp pointed teeth. It would seem probable that these ancestors were more or less bipedal, and adapted to live on dry land. They were probably much like the modern lizards in size, appearance and habitat:[2] From this ancestral type the Dinosaurs evolved into a great variety of different kinds, many of them of gigantic size, some herbivorous, some carnivorous; some bipedal, others quadrupedal; many of them protected by various kinds of bony armor-plates, or provided with horns or spines; some with sharp claws, others with blunted claws or hoofs. [Illustration: Fig. 6.--Outline Restorations of Dinosaurs. Scale about nineteen feet to the inch.] These various kinds of Dinosaurs are customarily grouped as follows: I. _Carnivorous Dinosaurs_ or _Theropoda_. With sharp pointed teeth, sharp claws; bipedal, with bird-like hind feet, generally three-toed;[3] the fore-limbs adapted for grasping or tearing, but not for support of the body. The head is large, neck of moderate length, body unarmored. The principal Dinosaurs of this group in America are _Allosaurus_, _Ornitholestes_--Upper Jurassic period. _Tyrannosaurus, Deinodon, Albertosaurus, Ornithomimus_--Upper Cretacic period. [Illustration: Fig. 7.--Skulls of Dinosaurs, illustrating the principal types--_Anchisaurus_ after Marsh, the others from American Museum specimens.] II. _Amphibious Dinosaurs_ or _Sauropoda_. With blunt-pointed teeth and blunt claws, quadrupedal, with elephant-like limbs and feet, long neck and small head. Unarmored. Principal dinosaurs of this group in America are _Brontosaurus_, _Diplodocus_, _Camarasaurus_ (_Morosaurus_) and _Brachiosaurus_, all of the Upper Jurassic and Comanchic periods. III. _Beaked Dinosaurs_ or _Predentates_. With a horny beak on the front of the jaw, cutting or grinding teeth behind it. All herbivorous, with pelvis of peculiar type, with hoofs instead of claws, and many genera heavily armored. Mostly three short toes on the hind foot, four or five on the fore foot. This group comprises animals of very different proportions as follows: 1. _Iguanodonts._ Bipedal, unarmored, with a single row of serrated cutting teeth, three-toed hind feet. Upper Jurassic, Comanchic and Cretacic. _Camptosaurus_ is the best known American genus. 2. _Trachodonts_ or _Duck-billed Dinosaurs_. Like the Iguanodonts but with numerous rows of small teeth set close together to form a grinding surface. Cretacic period. _Trachodon, Hadrosaurus, Claosaurus, Saurolophus, Corythosaurus, etc._