Actually, the pecans and hickories are part of the walnut family, Juglandaceae. There are many species within that family, and the pecan and hickory share the same genus, Carya. Just recently on a capitalization quiz, I included Carya tomentosa because I do like those tomentosa species--just fun to say. The walnuts within this family are in the various Juglans genera.

Although I've never seen anyone beat a hickory, I have a cross section of a hickory tree in which the tree has grown all around a nut. It is a very interesting cross section because it looks as though the nut grew inside the tree, but the most probable explanation is a squirrel found a hole in the tree, deposited the nut in the hole, and, over time, the tree grew around the nut in the little hole.