N. Keutgen, J.W.M. Jagt, P.J. Felder, and E.A. Jagt-Yazykova. “Stratigraphy of the Upper Vijlen Member (Gulpen Formation; Maastrichtian) in Northeast Belgium, the Southeast Netherlands and the Aachen Area (Germany), With Special Reference to Belemnitellid Cephalopods”. Netherlands Journal of Geosciences, vol. 89, Sept. 2010, pp. 109 -36, doi:10.1017/S0016774600000731.