Nautilus are cephalopods related to the Ammonites and look similar but their shells have less complex chambers. Whereas in Ammonites the walls that separate the chambers can be very complex and intrcate (sometimes resembling fractal patterns), Nautilus generally are plain and simple. Kind of like the Amish of the Cephalopod world. The specimen below is from Caen, France and is from the lower Jurassic (Toarcian stage). You can see that some of the replaced shell is still in place.
Thanks to Gery for this fine specimen.