Dallas--Temerlin McClain announced Wednesday that David Lyon has been named president of the advertising agency. Mr. Lyon, 39, formerly was group account director at Temerlin McClain and has been with the agency for eight years. Liener Temerlin remains chairman of the agency, and Dennis McClain remains CEO and executive creative director. In a statement, Mr. McClain said, ``This appointment signals Temerlin McClain's intent over time to have David as the primary leader of the agency.'' The agency's clients include Bell Helicopter/Textron, Nortel Networks and GTE. Temerlin McClain is a wholly owned subsidiary of True North Communications, Chicago.