N. Keutgen. “The Belemnite Zonation of the Uppermost Cretaceous in the Maastricht-Aachen-Liège, Brabant-Méhaigne and Mons Areas (Belgium, Southeast Netherlands)”. Netherlands Journal of Geosciences, vol. 90, Nov. 2011, pp. 165 -78, doi:10.1017/S0016774600001086.