Check Part VII: Jokes and the Comic in _Jokes and Their Relation to the
Unconscious_.  In the Norton standard edition (ISBN 0-393-00145-8)
reference to Twain is made on pp 230-1.  While I cannot summarize
Freud's specific or broader arguments, the reference begins:

   "An economy of pity is one off the most frequent sources of humorous
pleasure.  Mark Twain's humor usually works with this mechanism.

As I reread the passage, what is most interesting to me is that all
three of Freud's examples in this reference to Twain deal with writings
that deal in some way with Twain's family.  And two of the three begin
with Orion.

Neil Berkowitz