Will Clemens had two chances to prove a direct relationship to Sam Clemens:

CLEMENS FAMILY CHRONOLOGY, 1610-1912, published in 1914, which I've never 
seen cited by anyone.

THE GENESIS OF MARK TWAIN  a series of articles that he wrote for BIBLIO in 
1922-23, which also seems to be overlooked.

In neither of these genealogical works does he make the connection so far as 
I can tell, and if he'd had the proof I would think he would have published 
it in one or the other, or both.

I have capped  the title of the magazine article to a void apostrophes which 
upset the Forum listserver.

Kevin
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