“Over the Libyan Desert?” The other shook his head. “Never! Most of the Sahara is stone and rock, as here, but the Libyan Desert is sand, sand like the pictures you see of the desert, dunes from fifty to two hundred feet high, no water, no life, no vegetation. It is a waste as large as France and Germany together, where not a blade of grass grows, and where the only living things are creatures like the jerboas that have learned to do with so little water that a really good drink might kill them, and even they only live on the edges of that desert. No, Perry, you cannot explore that place, there would be no way to live.” The boy looked longingly at the southwestern horizon. “I’d awfully like to try,” he said. A slight and very hot puff of wind reached them, and, shading his eyes, Antoine looked anxiously at the distant dune hills. A thin curl of dust was rising from them. “The sand is blowing,” he said warningly; “we’ll go back.” It was a false alarm, however, for the wind died down, and before Antoine and Perry reached the camp again, the slight orange light that had overspread the sky had died away. The boy’s uncle greeted him with relief. “Glad to have you back,” he said, “I thought we were going to have a sand-storm, and that’s a thing it’s best not to be compelled to face. We’ve escaped so far; I hope our luck holds.” To himself Perry thought differently, he felt that he would not have had a real taste of the desert unless he had a chance to see one of the sand-storms of the Sahara, but, as the time drew near when the expedition was scheduled to return, he almost lost hope. The very week before the day set for leaving, Perry’s laborers unearthed the skull of a second Arsinotherium, a young bull, that must have stood nearly six feet at the shoulders, carrying four horns, one pair a foot in diameter at the base and three feet long. “Must have had some neck-muscles to carry those horns,” exclaimed Perry, gloating over his find, and watching the long and difficult job of packing the bones in plaster and huge wooden cases so that they might be loaded on camels and so that they might withstand transhipment across the sea. “Not only to carry them, but to use them,” commented his uncle. “Even Arsinotherium, big as he was, didn’t have everything his own way. There were Creodonts, such as the Pterodon, to worry him. They traveled in packs like jackals and the sharpest horns would be none too sharp for defense against a pack of those.” “Were they bigger than jackals, Uncle George?” “Yes, a little. But of course the biggest of the Creodonts were not as large as the great ‘sabre-tooth’ tigers of America and of Europe. Those are not found in this ancient African fauna. But you’ll have a chance to get acquainted with the sabre-tooths, Perry, when you come to do fossil-work in the States. There’s no lack of fossil beds there.” [Illustration: _Courtesy of American Museum of Natural History._ THE FOUR-HORNED GIANTS AT BAY.