Made a collegiate church in 1406, James Douglas, 1st Earl of Morton, enlarged it in 1477 and probably built the Scottish late-Gothic style chancel. Work on the chevet stopped in 1590 and the nave and transept remodelled in the mid-19C. The 16C Morton Monument, in memory of James Douglas and his wife, is located in the roofless choir. The effigies of the couple, worn smooth with the passage of time, show them in rich costume with lions at their feet.