"MARK TWAIN" delivered the opening lecture of the Young Men's Christian Association Course, on Wednesday evening last, in Armory Hall. He was introduced by himself, and his lecture "was intensely interesting to those who were interested." We are of the opinion of our young friend Crowninshield, at No. 3 Almy's Block, who is the sole agent for North Providence for "Mark's" new book, that "he is a fair lecturer, but he writes much better than he talks."