Wikipedia
Pachydiscus
Pachydiscus is an extinct genus of ammonite from the Upper Cretaceous with a world-wide distribution, and type for the desmoceratacean family Pachydiscidae. The genus' type species is P. neubergicus. Altogether some 28 species have been described.
The shell of Pachydiscus is compressed and high-whorled, with an oval or flat sided section. Ribs tend to be differentiated into short umbilical and separate ventro-lateral sets, with a smooth area between.
Pachydiscus includes two subgenera, P.(Pachysiscus) from the Campanian in which the ribs persist, and P (Neodesmoceras) from the Maastrichtian in which ribs disappear early, leaving an almost smooth shell.