Greca nodded soberly, keeping her eyes on the distant door to their rear. Twice that door had been opened, and twice she had directed the death rays into its opening to mow down the gangling figures behind it. But she had said nothing of this to her man. He was busy enough with his own task! "The door to the dome--" Dex shouted suddenly. But Brand merely nodded, even as a discharge from his tube annihilated the Rogan that had appeared in the doorway before them. He had seen that door stealthily opening even before Dex had. "It had better be soon, Dex!" he called. "Rogans in front of us--Rogans behind us--and--look out! On your side of the fence, there!" Dex whirled in time to pick off a grotesque, pipe-like figure that had suddenly appeared on the broad wall of the enclosure. Then he turned to the frenzied problem of driving the monster through the building wall. "The thing's going mad, Brand!" he cried, his voice high-pitched and brittle. "Watch out!" * * * * * It was only too evident that his statement was true. The baited monster, harried blindly this way and that, hounded against the blank wall behind it by something that bit chunks of living flesh out of its legs and sides, was losing whatever instinctive mental balance it had ever had. Its dimly functioning brain, probably no larger than a walnut in that gigantic skull, ceased more and more to guide it. With a rasping scream that set the Earthmen's teeth on edge, it charged for the wall on Dex's side. Dex just managed to swerve it with a blast from the tube so prolonged that half its great lower jaw fell away. At this the titanic thing went wholly, colossally mad! It whirled toward Brand, jerking around again as a searing on that side jarred its dull sensory nerves, then headed at last straight toward the stone wall of the dome building. With the rays from both tubes flicking it like monstrous spurs, it charged insanely toward the bulge of the circular wall. With all its tons and tons of weight it crashed against the stonework. There was a thunderous crackling noise, and the wall sagged in perceptibly, while the metal roof bent to accommodate the new curvature of its supporting beams. The monstrous lizard, jerked off its huge legs by the impact, staggered up and retreated toward the two men. But again the maddening pain in its hindquarters sent it careening toward the building wall. This time it raised high up on its hind legs in a blind effort to climb over it. "God, it must be five stories tall!" ejaculated Brand. Thunderingly its forelegs came down on the edge of the roof. * * * * * There was another deafening crash of stone and shrieking of torn metal. Just under the cornice, the wall sagged away from the roof and the top rows of heavy stone blocks slithered inward. "Again!" shouted Brand.