Your ammonite probably is...

Pachydiscus

Pachydiscus had a compressed shell that was evolute or slightly involute. It had short radial ribs and a ridge along its venter. Pachydiscus is found worldwide and lived in the Upper Cretaceous.

Public Domain, https://commons.wikimedia.org/wiki/File:Pachydiscus_neubergicus_Madagascar_Maastrichtian.JPG