Dian of the Lost Land — Edison Marshall

Adam knew that the creatures who surrounded him had vanished from the earth five million years ago. Their beautiful art, their terrible weaponry, their religion, customs and language, even the animals they hunted and the alien dark tribe against whom they waged eternal war, all belonged to a time before the great glaciers had retreated and man had assumed control of the globe. But Adam was in no position to deny their reality. For now he and his companion, the renowned Professor Karl Belgrade, were being led to their ruler -- the young and beautiful Queen who would decide whether the intruders would live or die.