This is the Rissoa costulata of Searles Wood from the Coralline Crag. The costulata of Risso, the one of Alder, are other species. Origin of the name: « In the collection of General Stefanis at Naples I noticed R. costulata of S. Wood from Sicily; it may be called Stefanisi. » – J. G. Jeffreys: British conchology vol. V, London 1867, via BHL.