From the Jurassic,Kimmeridgian stage.
Collected from Abingdon,Oxford,England.
This is a Very rare ammonite Aptychi or Operculum as they are sometimes known
The exact function is still debated, but it is considered to be either part of the Jaw,
or a door to close off the aperture of the shell opening, along with a second Aptychi for left and right!
The specimen is loose from Kimmeridge clay and measures 41mm x 32mm.