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aptychus A calcitic plate associated with Mesozoic ammonites. Normally these plates occur in pairs. Aptychi are shaped like bivalves (Bivalvia) and have an ornamented outer surface. Since they have been found inside the aperture of ammonoid shells they were originally interpreted as opercula, protecting the ammonoid body when it was withdrawn into the shell. It has now been shown that aptychi are probably the lower jaws of ammonoids and that the horny upper jaw is rarely preserved.