From the back of the book:
On Caprona, the Land That Time Forgot, all of the world's savage past still lived. Here were dinosaurs and flying reptiles, here were the most primitive of cavemen and the last of Bronze Age barbarians. But there was one more secret that the claws and fangs and sharp-edged spears guarded most of all.

This is the story of the man who tried to find that final secret. When Bradley, the adventurer, dared to cross the last terrible barrier to the heart of Caprona, he entered a world of wonder, terror and danger beyond the imagination of any man - except the imagination of Edgar Rice Burroughs, author of Out Of Time's Abyss.

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IN the lost valleys of Caspak, a land forgotten by time and shielded form the world by uncharted seas and towering cliffs, a group of marooned adventurers struggled for survival.

The English seaman Bradley, entering Caspak to search for the lost explorers, found a world of terror and wonder beyond imagination. Flying reptiles battled Bronze Age barbarians, while death took a thousand terrible forms in the primeval jungles. But when Bradley dared to enter the hidden heart of the mysterious continent, he stumbled upon Caspak's final and most terrible secret. 

There, in a city of hte damned, where teh streets were paved with human skulls, Bradly found forbidden love - and faced the deadliest peril of all ...

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AIR RAID

Bradley felt gooseflesh  rise on  his body, and it was with difficulty that he refrained from firing upon nocturnal intruder. While his attention was riveted on the grim, winged thing circling before him, there swooped silently out of the black night behind him another ghostly shape.
So great was the impact when the thing struck Bradley between his shoulders, that he was half stunned. His rifle flew from his grasp. He felt claw-like talons seize him and sweep him off his feet. A cry of warning was forced back into his lungs as he was borne rapidly upward into the inky sky ...