Starting with the description of a caledonian type-section south of Nasafjäll, Lapland, some ordovician evolutionary trends are discussed. The discrimination of a new middle-ordovician series, the Sarvas Series, is proposed. It answers roughly to the lower Hovin Series of the Trondhjem Region and stands between the Mesket and Gilliks Series of Västerbotten. The position of ordovician volcanics is discussed.