Aptychus
Sign in to savethumb|upright 2.0|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. thumb|One of what would have been a pair of aptychi (at first given the name Trigonellites|Trigonellites latus and described as a bivalve) from the [[Kimmeridge Clay Formation in England]]
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thumb|upright 2.0|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. thumb|One of what would have been a pair of aptychi (at first given the name Trigonellites|Trigonellites latus and described as a bivalve) from the [[Kimmeridge Clay Formation in England]]
An aptychus is a type of marine fossil. It is a hard anatomical structure, a sort of curved shelly plate, now understood to be part of the body of an ammonite. Paired aptychi have, on rare occasions, been found at or within the aperture of ammonite shells. The aptychus was usually composed of calcite, whereas the ammonite shell was aragonite.