Tarzan sat in a brown study for a long time after he finished reading the letter. It was filled with so many new and wonderful things that his brain was in a whirl as he attempted to digest them all.
So they did not know that he was Tarzan of the Apes. He would tell them.
In his tree he had constructed a rude shelter of leaves and boughs, beneath which, protected from the rain, he had placed the few treasures brought from the cabin. Among these were some pencils.
He took one, and beneath Jane Porter's signature he wrote: I am Tarzan of the Apes

Ch. 18 : The Jungle Toll - Tarzan of the Apes (1912)