The body of a 78-year-old man was found on a bus at South Station Wednesday, but the death does not appear suspicious, officials said. The man’s body was discovered in his seat on a private coach that arrived at about 10 a.m., said Jake Wark, a spokesman for the Suffolk district attorney. The man, who was from Vermont, boarded the bus in Lebanon, N.H., and was pronounced dead at South Station, Wark said. Authorities saw no sign of altercation or trauma, he said. The district attorney’s office is withholding the man’s name because no crime is suspected, Wark said.