	Authors	Year	Title
	Bisat	1924	The Carboniferous goniatites of the North of England and their zones. Proceedings of the Yorkshire Geological Society, 20: 40-124, pl.1-10; Leeds.
	Ramsbottom & Calver	1962	Some marine horizons containing Gastrioceras in North West Europe. Quatrième Congrès International de Stratigraphie et de Géologie du Carbonifère, Heerlen 1958, Compte Rendu 3: 571-576, 2 figs., pl.14, 15, Heerlen.