Zenostephanus
Zenostephanus is true ammonite genus from the lower part of the Upper Jurassic of eastern England and Scotland belonging to the ammonitid family, Perisphinctidae. Zenostephanus is revised from Xenostephanus, a  preoccupied name for a new genus of toxodont notoungulate from Argentina. The type is Aulacostephanus ranbyensis

References

 Perisphinctidae in the Treatise on Invertebrate Paleontology,  Part L, Mollusca 4. Ammonitida genera
Perisphinctidae