This one's a headache to all collectors, as it is BOTH the 4th volume of the Inner Earth series and the 13th of Tarzan. An urgent message from Pellucidar, world of primitive men and savage jungles inside the crust of the Earth, called upon Tarzan for assitance. The Apeman arranges an expedition, travelling via the great dirigible O-220 through the Polar opening to the inner world.