Francis Robaszynski, and Walter Kegel Christensen. “The Upper Campanian-Lower Maastrichtian Chalks of the Mons Basin, Belgium: A Preliminary Study of Belemnites and Foraminifera in the Harmignies and Ciply Areas”. Netherlands Journal of Geosciences, Jan. 1989, pp. 391-08, https://njgjournal.nl/index.php/njg/article/view/12920.