Aptychus

It is a hard anatomical structure, a sort of curved shelly plate, now understood to be part of the body of an ammonite.

This circumstance led to them being initially classified as valves of bivalves (clams), which they do somewhat resemble.

Aptychi seem to have most often existed as bilaterally-symmetrical pairs and were first described (incorrectly) as being the valves of bivalve mollusks.

Aptychi are now considered to be a two-valved closing hatch on the shells of extinct ammonites or a double-plate jaw-piece similar to that of some modern cephalopods.

[2][3][4][5] Set near to or against the shell's terminal opening (the living chamber), the aptychi usually consisted of two identical but mirror image valves.

Some examples of aptychi (top right: Oppelia from Late Jurassic of Solnhofen, Germany; bottom left: aptychi (recto and versus) from Late Jurassic of Lombardy, Italy), and conceptual scheme of their function if indeed they were used to close the shell aperture, as opposed to being jaws.
One of what would have been a pair of aptychi (at first given the name Trigonellites latus and described as a bivalve) from the Kimmeridge Clay Formation in England