Your ammonite probably is...
Titanites
Titanites was a giant Upper Jurassic ammonite from Europe and possibly other Arctic regions. Titanites was covered in packed radial ribs and lacked a ventral keel or groove. It was narrow. The inner coils of titanites had branched radial ribs.
Image by Leptictidium, CC BY-SA 3.0 Unported, https://commons.wikimedia.org/wiki/File:Titanites_giganteus.PNG