Aptychi are pairs of shell-like calcareous valves (opercula) closing the aperture of ammonoid cephalopods. A single specimen is called an anaptychus. Aptychi are often found inside the ammonoid shell and may have functioned as jaws and. When found away from ammonites they are easily mistaken for the shells of bivalves. This fossil is from Upper Jurassic sediments near Geisingen, S. Germany and its actual length is 50 mm