* * * * * Slowly, an inch at a time, the doctor crept forward, hand-grenades in readiness. Presently he reached the crack and, shifting one of the grenades into his pocket, he drew forth an electric torch and sent a beam of light through the crack into the dark interior of the earth. For a moment he stood thus, and then suddenly snapped off his torch and straightened up in an attitude of listening. The straining ears of Carnes and Lieutenant Pearce could hear a faint slithering noise coming toward them, not from the direction of the crack, but from the interior of the cave. Simultaneously a faint, musky, reptilian odor became apparent. "Run!" shouted the doctor. "Run like hell! It's loose in the cave!" The lieutenant turned and fled at top speed toward the distant entrance to the cave, Carnes at his heels. Dr. Bird paused for an instant, straining his ears, and then threw a grenade. A blinding flash came from the point where the missile struck and a white cloud rose in the air. The doctor turned and fled after his companions. Not for nothing had Dr. Bird been an athlete of note in his college days. Despite the best efforts of his companions, who were literally running for their lives, he soon caught up with them. As he did so a weird, blood-curdling screech rose from the darkness behind them. Higher and higher in pitch the note rose until it ended suddenly in a gurgling grunt, as though the breath which uttered it had been suddenly cut off. The slithering, rustling noise became louder on their trail. "Faster!" gasped the doctor, as he put his hand on Carnes' shoulder and pushed him forward. * * * * * The noise of pursuit gained slightly on them, and a sound as of intense breathing became audible. Dr. Bird paused and turned and faced the oncoming horror. His electric torch revealed nothing, but he listened for a moment, and then threw his second grenade. Keenly he watched its flight. It flew through the air for thirty yards and then struck an invisible obstruction and bounded toward the ground. Before it struck the downward motion ceased, and it rose in the air. As it rose it burst with a sharp report, and a wild scream of pain filled the cavern with a deafening roar. The doctor fled again after his companions. By the time he overtook them the entrance of the cave loomed before them. With sobs of relief they burst out into the open. The guards sprang forward with raised rifles, but Dr. Bird waved them back. "There's nothing after us, men," he panted. "We got chased a little way, but I tossed our pursuer a handful of phosphorus and it must have burned his fingers a little, judging from the racket he made. At any rate, it stopped the pursuit." The major hurried up. "Did you see it, Doctor?" he asked.