Phylloceras inflatum fossil from the Mahajanga Province of Madagascar. The fossil dates to the Albian Stage of the Cretaceous Period, around 105 Ma (mega-annum, or million years). This ammonite is a shelled cephalopod related to extant squids.
The septum on this ammonite is the original nacre (aragonitic mother of pearl). Orange calcite crystals are in the chambers of the ammonite.