Complex sentence: Under Riddle's guidance, the university resumed classes on October 8th, 1855, and chose his successor, John F. McLaren, who was formally appointed as the fourth permanent head of the university on December 19,1956.
Split into simpler sentences: Under Riddle's guidance, the university resumed classes on October 8th, 1855. Riddle continued to help oversee the university until John F. McLaren was formally appointed as the fourth permanent head of the university on December 19,1956.