I apologize for the darker than usual photos of this specimen but I must have taken them on a cloudy day. they are of an ammonite called Trimarginites trimarginatus that comes from the Oxfordian stage sediments (Jurassic) near Poitiers, Vienne Department, France. The whorls are wide and overlap each other quite a bit so that there is just a small area in the center that gives evidence of previous shell. The cross section of the shell is flat.
The sutures are very complex. Here is a closeup of the shell and the visible sutures...
..and an image of the pattern lain flat.
The Ammonites.fr website has a page on this species that can be found here. Thanks to Gery for the fossil!