Sorted in the order in which they were published.
The Caspak Series is a trilogy of books, all published in 1918, featuring the island of Caprona in a 'lost world' saga. The island is also called Caspak by its native humanoid inhabitants – thus the name of the trilogy. In the first novel, Caprona is described as a land mass near Antarctica and was first reported by the (fictitious) Italian explorer Caproni in 1721, the location of which was subsequently lost. The island is ringed by high cliffs, making it inaccessible to all but the most intrepid explorers, and has a tropical river teeming with primitive creatures that are elsewhere extinct.