188 WHEN THE WORLD WAS YOUNG
they were most like crocodiles, hut in other respects, again, they remind us of ostriches. To this class Naturalists have given the name of Dinosaurs, from two Greek words, which mean ' terrible lizards.'
All the tribe were alike in one way, for they had four legs ; but in some the structure of the bones shows that the Dinosaur could, when it chose, stand upright, while other varieties, such as the Brontosaur, must have been compelled, or at any rate must have preferred, to walk on all fours. This monstrous beast was about sixty feet long, its skeleton has always been found on the bank of a lake or river, and it probably fed on water plants. It had a long neck, which would enable it to rear its head out of the water and see whether the coast was clear of its enemies, and a long tail, which was a great help in swimming. But when on land it must have been difficult indeed for an animal of such huge bulk to get out of the way when attacked, and still more difficult for it to escape detection, as every one of its tracks measures a whole square yard.
About the same time that the Brontosaurus was wallowing among the reeds of the lakes and rivers which covered the tract of country now called Colorado, one of his distant cousins might have been met with any day in the Weald of Sussex, had there been anyone living on the earth to take a walk! This particular reptile has been given the name of Iguanodon, from a peculiarity of its teeth. The largest kind known is thirty feet long, from its nose to the end of its powerful tail, and when walking, as it always did, on its hind legs, was as tall as a very big elephant. "From the hollowness of its limb bones it was able to move more lightly than some of the other animals whose bones were solid throughout, and this was very necessary, as, unlike many of these old lizards, the Iguanodon had no sharp knots or spines on its skin to ward off the attacks of its flesh-eating foes. So, when standing as high as it did, it saw one of these huge