Lower Ordovician trilobites of the Kirtonryggen Formation, Spitsbergen Fortey Richard A. Heftet / 2013 / Engelsk
Arenig Series in South Wales Stratigraphy and Palaeontology Fortey Richard A. Heftet / 1987 / Engelsk
Revised Correlation of Ordovician Rocks in the British Isles Fortey Richard A. Heftet / 2000 / Engelsk
