Published online by Cambridge University Press: 06 July 2012
The type material of Orthocera gigantea J. Sowerby 1819 is refigured and redescribed. Specimens referred to this species by J. Phillips, J. Fleming, F. McCoy and F. S. Wallis have similarly been reinvestigated. The counterpart of the type of Actinoceras simmsii C. Stokes 1840 is also illustrated. Numerous other specimens from England, Scotland and Ireland have been studied. The material is found to be referable to the genus Rayonnoceras Croneis and to embrace a number of species. The calcification of the siphuncular structures is found to be variable in this genus.