Involuticeras: a perisphinctacean ammonite

Ammonitina

The Ammonitina comprises a diverse suborder of ammonoid cephalopods that lived during the Jurassic and Cretaceous periods of the Mesozoic Era. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.The shells of...

 belonging to the Aulacostephanininae from the Upper Jurassic of Europe and possibly Mexico. Shell involute, moderately compressed,venter rounded, ribbed. Similar in general form to Aulacostephanus

Aulacostephanus

Aulacostephanus is an extinct ammonoid cephalopod genus from the Upper Jurassic Kimmeridgian belonging to the perisphinctacean family Aulacostephanidae....

and Epicephalites

Epicephalites

Epicephalites is a perisphictid ammonite, included in the subfamily Aulacostephaninae, from the Upper Jurassic of New Zealand and Mexico, related to Involuticeras. Its shell is involute, whorls inflated with a deep umbilicus...

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Treatise on Invertebrate Paleontology, Part L,(1957), Mesozoic Ammonoidea, p. L324.