Although some of these Dinosaurs were no larger than chickens, others were by far the largest creatures that ever were, on sea or land. Many of the biggest lived on grass, just like an old cow, while the flesh-eating Dinosaurs lived on them. Some of these Dinosaurs went on all fours, while others ran about on their hind legs, and when they stood still, propped themselves up on their big, thick tails as do kangaroos. The Camptosaurus, one of whose favorite resorts was the land that is now Wyoming, was thirty feet long. Another called the Brontosaurus, was sixty feet long. The Atlantosaurus, one of the pioneers of Colorado, measured eighty feet from the end of his nose to the end of his tail, and all of them were built in proportion. The Stegosaurus, also an early settler in Wyoming, had huge bony plates, like ploughshares, sticking out all along his back from the nape of his neck to the end of his tail. He seems to have gone about looking quite ugly and humpbacked, as our old cat does when she has words with the dog. After the swamps dried up and the lizards could no longer make a living, came the reign of the mammals; including the Mastodons and the Mammoths, marching in countless herds, trumpeting through the forests. HOW SOME MONSTERS PLOUGHED THE FIELD But besides what they did in the way of fertilizing the land with their flesh and bones some of the mammals did a good deal of ploughing. Among these early ploughmen were the Mastodons and the Mammoths, and another elephant-like creature with two tusks, that he wore, not after the fashion among elephants to-day, but curving down from his chin, somewhat like Uncle Sam's goatee. He used these tusks, it is supposed, not only for self-defense, but for grubbing up roots which he ate. If so, they must have been about as good ploughs as those crooked sticks that were used by the early farmers among men, and that are still in use among primitive peoples. THE ELEPHANT FAMILY AS PLOUGHMEN What makes it more likely that the creature with the down-curving tusks stirred the soil with them is that his cousins, the elephants of to-day, are themselves great ploughmen. Elephants feed, not only on grass and the tender shoots of trees, but on bulbs buried in the soil, which they hunt out by their fine sense of smell. In digging these bulbs they turn up whole acres of ground. Elephants also do a great deal of ploughing by uprooting trees so as to make it more convenient to get at their tender tops. Sir Samuel Baker, the explorer, says the work done by a herd of elephants in a mimosa forest in this way is very great and that trees over four feet in circumference are uprooted. In the case of the biggest trees several elephants work together, some pulling the tree with their trunks, while others dig under the roots with their tusks. To be sure, the mimosa-trees have no tap roots, but tearing them out of the ground is no small job, nevertheless. It takes strength and it takes engineering.