Complex sentence: In retirement he served as president of the Royal Historical Society from 1961 to 1965 and was awarded the Cymmrodorion medal on his eightieth birthday in 1971 to mark his service to Wales.
Split into simpler sentences: In retirement he served as president of the Royal Historical Society from 1961 to 1965. He was awarded the Cymmrodorion medal on his eightieth birthday to mark his service to Wales.