N. Keutgen, J.W.M. Jagt, P.J. Felder, and E.A. Jagt-Yazykova. 2010. “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 89 (September):109 -36. https://doi.org/10.1017/S0016774600000731.