The first of four volumes of revised Part L. Prepared by C. W. Wright, with sections by J. H. Calloman and M. K. Howarth. The revisions replace the original Part L, first published in 1957. Volumes 1 to 3 in the revised Part L series will follow as they are completed. They will cover such introductory material as morphology, biostratigraphy, and classification and will present diagnoses, stratigraphic ranges, and illustrations of Paleozoic to Jurassic ammonoids in traditional Treatise style. Past volumes of the Treatise dealt with taxa throughout their ranges rather than with fossils from restricted intervals of geological time. It is possible to treat the ammonoids stratigraphically because few genera from the Jurassic ranged into the Cretaceous, and all ammonoids became extinct by the end of the Cretaceous.