Today’s tidbit is that the 1937 Hobbit has “durstn’t”  and the 1951 has “dursn’t“. I am not finding any reference at all on the negative contraction, but “durst” is given as the current past form, alongside “dared”.  Google’s Ngram Viewer tells us that both “durstn’t” and “dursn’t” fist appear in their corpus in the early 1800s, at first about equal in frequency but by 1925, “dursn’t” about twice as frequent as “durstn’t”.  “Did not dare” overwhelms these forms by an order of magnitude, and is a much older phrase than these contractions are.  Present forms “dassn’t” and “dare not” are more common than all those others, the latter much more popular of the two.

See more our earlier discussion of Gollum’s speech.

“Dare” is uncommon as are the contractions.

• 05.124 ‘But we dursn’t go in
• 05.124 no we dursn’t.

“dare, v.1.” OED Online. Oxford University Press, June 2015. Web. 18 June 2015.