shares of arthur j gallagher  co ajg slid 341 to 17709 wednesday on what proved to be an allaround grim trading session for the stock market with the sp 500 index spx falling 070 to 389193 and dow jones industrial average djia falling 087 to 3187457 arthur j gallagher  co closed 2528 below its 52week high 20237 which the company achieved on january 27th the stock underperformed when compared to some of its competitors wednesday as marsh  mclennan cos mmc fell 202 to 15408 trading volume