James R. Mellor was elected Wednesday as president and chief operating officer of General Dynamics Corp., effective Jan. 1. Mellor, 60, is executive vice president of marine, land systems and international at General Dynamics corporate offices in St. Louis. He will succeed President and CEO Herbert F. Rogers, 65, who will continue on special assignment as general manager of the company's Fort Worth Division. Rogers will become vice chairman on Jan. 1. William A. Anders, General Dynamics' vice chairman, will succeed Stanley Pace as chairman and CEO when Pace retires at the end of the year. Mellor joined General Dynamics in 1981 as executive vice president of commercial systems and corporate planning and as a member of the board. Before joining General Dynamics he was president and chief operating officer of AM International Inc.