 Some ammonites have been found in association with a single horny plate or a pair of calcitic plates.In the past, these plates were assumed to serve in closing the opening of the shell in much the same way as an operculum, but more recently, they are postulated to have been, instead, a jaw apparatus.The plates are collectively termed the aptychus or aptychi in the case of a pair of plates, and anaptychus in the case of a single plate.