Ynze A. Baumfalk, and Frans Willemsen. “Ecophenotypic Variation of the Larger Foraminifer Orbitoides Apiculata from the Maastrichtian Stratotype”. Netherlands Journal of Geosciences, Jan. 1986, pp. 23-34, https://njgjournal.nl/index.php/njg/article/view/13126.