Name: Scaphites nodosus
Age: Upper Cretaceous, Maastrichtian
Provenance: Pierre, South Dakota, USA

Ammonite is 6.2 cm.
As can be seen, this specimen is on its gangue. There are some gaps at the ventral area (see photos) and I think the ammonite has been glued back on the stone.
The front side is nevertheless extremely well preserved. It is a beautiful sample of an unusual ammonite.