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. Apty...
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http://en.wikipedia.org/wiki/Aptychus
• (n.) A shelly plate found in the terminal chambers of ammonite shells. Some authors consider them to be jaws; others, opercula.
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http://thinkexist.com/dictionary/meaning/aptychus/
Ap'ty·chus noun [ New Latin , from Greek
'a priv. + ..., ..., fold.]
(Zoology) A shelly plate found in the terminal chambers of ammonite shells. Some authors consider them to be jaws; others, opercula.
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http://www.encyclo.co.uk/webster/A/111
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