1: Walter Blair: The Literature Of United States(ed.). p. 747.
2: The Phrenological Journal, April 1910.
3: Madeleine B. Stern; “Mark Twain had His Head Examined”; American Literature (Man 1969),pp.207-218
4: Bellamy: Mark Twain as a Literary Artist, p. 68
5: HenryNase Smith; The Ordeal of Mark Twain; pp,13-14
6: Samuel C. Webster: Mark Twain, Business Man, Bostan, 1946, p. 317