- A marine fossil comprising a difficult anatomical structure like a curved shelly dish, once part of the human anatomy of an ammonite.
- A shelly plate found in the terminal chambers of ammonite shells. Some authors give consideration to them is jaws; other individuals, opercula.
- In Cephalopoda, a plate formed of a shelly compound, found in the critical chamber of particular fossil mollusks, as ammonites, and regarded by some as an operculum.
Etymologically Related for "aptychus"
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