I. Walaszczyk, J.W.M. Jagt, and N. Keutgen. “The Youngest Maastrichtian ‘true’ Inoceramids from the Vijlen Member (Gulpen Formation) in Northeast Belgium and the Aachen Area (Germany)”. Netherlands Journal of Geosciences, vol. 89, Sept. 2010, pp. 147 -67, doi:10.1017/S0016774600000755.