The Tarzan Twins

Two schoolboys, Dick and Doc, are cousins who resemble each other because their mothers are twins. As Dick is also related to Tarzan through his father, they become known as the Tarzan Twins. 

Invited to visit Tarzan's African estate, they become lost in the jungle and are imprisoned by cannibals, from whom they escape. They are then reunited with their host, who introduces them to his pet lion, Jad-bal-ja. 

Subsequently, they become involved in an adventure involving exiles from the lost city of Opar, who have kidnapped Gretchen von Harben, the daughter of a missionary. 

The Tarzan Twins, like all well-behaved twins, were born on the same day and, although they were not as "alike as two peas," still they resembled one another quite closely enough to fulfill that particular requirement of twinship; but even there they commenced breaking the rules that have been governing twins during the past several millions of years, for Dick had a shock of the blackest sort of black hair, while Doc's hair was the sunny hue of molasses candy. 

Then it was that the big surprise came in a letter that Dick received from his mother. Tarzan of the Apes had invited them all to visit him and spend two months on his great African estate! 

The boys were so excited that they talked until three o'clock the next morning and flunked in all their classes that day.