	Authors	Year	Title
	Hodson & Leckwijck	1958	A Namurian Marker-Horizon at Büsbach, near Aachen, Western Germany. Association pour l'Etude de Paléontologie et de la Stratigraphie Houillères, Publication 35: 1-13, fig.1-3, pl.A-B; Bruxelles.
	Moore	1946	The Carboniferous goniatite genera Girtyoceras and Eumorphoceras. Proceedings of the Yorkshire Geological Society, 25: 387-445, 31 figs., pl.22-27; Wakefield.