Article: Ammonites polyopsis Dujardin, 1837 and the Cretaceous ammonite family Placenticeratidae Hyatt, 1900
Ammonites polyopsis Dujardin, 1837 is referred to the genus Placenticeras Meek, 1876 and shown to have priority over Ammonites ribourianus d'Orbigny, 1850, Placenticeras depressum Hyatt, 1903, P. grossouvrei Hyatt, 1903 (non Semenov, 1899) (renamed P. hyatti Diener, 1925), P. incisum Hyatt, 1903, P. schlueteri Hyatt, 1903, and P. crassatum Hyatt, 1903 amongst others. This plethora of names results from early failure to recognize the strong dimorphism present in the species. Recognition of this dimorphism and the wide intraspecific variation seen in P. polyopsis provides a basis for a review of the Placenticeratidae and its constituent genera, of which revised diagnoses and synonymies are presented. An effect of the nomenclatorial results of the study is that the widely cited. P. syrtale Zone should be renamed the P. polyopsis Zone.