As the author of most of the "dirty" entries in the _Mark Twain
Encyclopedia_, I guess I ought to weigh in here.  I don't have my copy here
at home, but I do have my rough draft.  The earliest publication attributed
it to Petroeum V. Nasby, but, to quote myself, "most scholars generally
accept MT's authorship."

I agree with Larry M. that "Onanism" is much more clever.  And I like
"1601" best of all these bawdy, scatological pieces.  I remember my glee
when I first found it in the library.  I ended up doing my M.A. thesis on
it (which is why I guess I ended up covering all the "dirty" work for the
encyclopedia), and I am probably semi-legendary at Appalachian State U. for
uttering the phrase "art for fart's sake" during my thesis defense in
answer to a question about the work's importance.  And now I get to say it
again in public!  Tee hee--probably very much MT's impulse for writing
these little diversions.

John Bird