Out of Time's Abyss by Edgar Rice Burroughs, Science Fiction

This is the tale of Bradley after he left Fort Dinosaur upon the west coast of the great lake that is in the center of the island. Upon the fourth day of September 1916, he set out with four companions through the heavy Caspakian air. About them upon the ground, among the trees and in the air over them moved and swung and soared the countless forms of Caspak's teeming life. They had taken a dozen steps, perhaps, when a hideous growl broke from a dense thicket ahead, the leafy branches parted, and the hideous head of a gigantic bear emerged. "Lie still!" shouted Bradley. "Can't waste ammunition." Like a bolt of lightning the bear flashed down upon the Englishman. "Now run!" Bradley called to Tippet and himself turned in flight toward a nearby tree. The other men, now safely ensconced upon various branches, watched the race with breathless interest. Would Bradley make it? It seemed scarce possible. And if he didn't. . . !