It is only about 15,000,000 years ago, for example, that the biggest of them all, the Dinosaurs, lived, while the earth itself is now supposed to be some 100,000,000 years old. Their numbers were enormous, and it is probable there is not an acre of ground from the Atlantic to the Pacific, and from Alaska to the tip end of South America that has not been fertilized by their bones. In fact, of certain species I have found the bones scattered all the way from Oregon to Patagonia; so this must have been their pasture. They were not only all over the land, but in the lakes and in the great sea that once extended right through North America from the Gulf of Mexico to the Arctic Ocean. And they were along the shores of the sea and in the swamps. The bones of the ancestors of the whale were found in such quantities in some of the Southern States that they were used to build fences until it was found they were much more valuable to enrich the fields themselves. [Illustration: THE HEAD OF HESPERORNIS "Then there was a great toothed, diving creature with wings. They've named him the Hesperornis, which means 'western bird,' because the fossils of the best-known species were found in the chalk-beds of Kansas."] In the great American inland sea of those days swam one kind of fierce fish-lizard that took such big bites he had to have a hinge in his jaw. Because of this hinge he could open his mouth wider without putting anything out of place, don't you see? He was called the Mesosaur. But he never bit the Archelon, who was in his crowd, because he couldn't. The Archelon was the king of turtles, and, like all the turtle family, wore heavy armor. He was over twelve feet long. And sharks--no end of them! A shark at his best is bad enough, but the sharks of those days were almost too terrible to think about. Such jaws! And teeth like railroad spikes! Then there was a great toothed diving creature with wings. They've named him the "Hesperornis," which means "western bird." He was given the name because the fossils of the best-known species were found in the chalk-beds of Kansas. [Illustration: GREATEST OF ANCIENT FLYING MACHINES Mr. Pterodactyl, on his way to dinner, looked like this. He was the largest of all flying-machines before the days of the Wright brothers. He would have measured--if there had been anybody to measure him--twenty feet across the wings! Like the Hesperornis, he always dined on fish.] Over the waters flew another bird-like, fish-like, bat-like thing called the Pterodactyl. Look at his picture and you will see how he got his nickname. It means "finger-toe." He was the largest of all flying-machines until the days of the Wright brothers. It was over twenty feet across his wings, from tip to tip; and, like the Hesperornis, he always had fish for dinner. [Illustration: A BIG "LITTLE FINGER" AND WHAT IT WAS FOR