Amazon.com description: Product Description: "Whoa!"—"whoa!" With quick jerks on their bridle reins Thure Conroyal and Bud Randolph pulled up their horses and listened shiveringly. Again that same shrill whistling scream of dreadful agony and fear, that had caused them to rein up their horses so suddenly a moment before, came from the valley beyond the brow of the little hill up which they had been slowly riding, and chilled the very marrow in their bones with the terrible intensity of its fear and anguish. Then all was still. "What—what was it?" and Thure turned a startled face to Bud. "It didn't sound human and I never heard an animal scream like that before. What can it be?" "I don't know," Bud answered, his face whitening a little; "but I am going to find out. Come on," and, swinging his rifle into position where it would be ready for instant use, he started up the hill, his eyes fixed in the direction whence had come those fearful screams.