On the Lower Kimeridgian Ammonite Genera Pictonia, Rasenia, Aulacostephanus and Ataxioceras

Abstract

The four genera discussed below are mentioned in almost every paper, memoir, and monograph dealing with the Lower Kimeridge Clay in this country and abroad. The first three were given currency in the literature of the Upper Jurassic when species belonging to them were chosen as index-fossils for the three lowest zones of the Kimeridge Clay by Salfeld (1913–14). Ataxioceras, though not comprising any zonal index fossils, is almost as important and has been repeatedly cited for 56 years from all parts of the world.