Another aristocratic marriage has hit the rocks. Lord Snowdon, former husband of Princess Margaret, and his wife, Lucy, are undergoing a trial separation. The countess, who married the photographer Lord in 1978 after the end of his 18-year marriage to the Queen’s sister, moved out of their Kensington home a few weeks ago and is now renting a house from a female friend. A statement by Countess Snowdon said the parting was amicable. Two years ago, the British tabloids revealed that Snowdon had been conducting an affair throughout his marriage with Ann Hills, who committed suicide on Dec. 31, 1996. Snowdon, the father of Viscount Linley and painter Lady Sarah Chatto, has made no comment on the separation.