What is "grammoceras"

Grammoceras is an extinct ammonite found in Yorks Ravenscar, England, from Jurassic period sediments. Its overall distribution is fairly world wide.

Grammoceras has a thin, discoidally evolute shell with simple, gently sigmoid ribbing and a low ventral keel. Dumortiera and Catulloceras from the same age are similar except that Dumortiera has a more rounded venter and Catulloceras has a subquadrate whorl section. Asthenoceras from the slightly younger lower Bajocian may be a subgenus.