«The People That Time Forgot» by Edgar Rice Burroughs

After Bowen Tyler went missing and sent a message in a bottle letting the world know he was marooned on the legendary island of Caprona, his faithful secretary and friend, Tom Billings organizes a rescue expedition. They find Caprona, Billings makes it onto the island, and things go wrong immediately. The second of three stories about the lost world of Caspak.