L. Witte, and H. Schuurman. “Calcareous Benthic Foraminifera across the Cretaceous/Tertiary Boundary in the Geulhemmerberg (SE Netherlands)”. Netherlands Journal of Geosciences, vol. 75, Jan. 1996, pp. 173-86, https://njgjournal.nl/index.php/njg/article/view/12370.