▁more ▁recently , ▁they ▁are ▁post ulated ▁to ▁have ▁been , ▁instead , ▁a ▁j aw ▁appar atus . ▁ ▁The ▁pl ates ▁are ▁collect ively ▁term ed ▁the ▁a pty chus ▁or ▁a pty chi ▁in ▁the ▁case ▁of ▁a ▁pair ▁of ▁pl ates , ▁and ▁an apt ych us ▁in ▁the ▁case ▁of ▁a ▁single ▁plate . ▁The ▁pa ired ▁a pty chi ▁were ▁symmetric ▁to ▁one ▁another ▁and ▁equal ▁in ▁size ▁and ▁appearance . ▁ ▁An apt ych i ▁are ▁relatively ▁rare ▁as ▁foss ils . ▁They ▁are ▁found ▁representing ▁am mon ites ▁from ▁the ▁Dev onian ▁period ▁through ▁those ▁of ▁the ▁C ret ace ous ▁period . ▁ ▁Cal c ified ▁a pty chi ▁only ▁occur ▁in ▁am mon ites ▁from ▁the ▁Mes oz o ic ▁era . ▁They ▁are ▁almost ▁always ▁found ▁det ached ▁from ▁the ▁shell , ▁and ▁are ▁only ▁very ▁rarely ▁preserved ▁in ▁place . ▁Still , ▁sufficient ▁numbers ▁have ▁been ▁found ▁closing ▁the ▁a pert ures ▁of ▁foss il ▁am mon ite ▁shell s ▁as ▁to ▁leave ▁no ▁doubt ▁as ▁to ▁their ▁identity ▁as ▁part ▁of ▁the ▁an atom y ▁of ▁an ▁am mon ite . ▁ ▁Lar ge ▁numbers ▁of ▁det ached ▁a pty chi ▁occur ▁in ▁certain ▁b eds ▁of ▁rock ▁( such ▁as ▁those ▁from ▁the ▁Mes oz o ic ▁in ▁the ▁Al ps ). ▁These ▁rocks ▁are ▁usually ▁accum ulated ▁at ▁great ▁depth s . ▁The ▁modern ▁N aut ilus ▁la cks ▁any ▁cal cit ic ▁plate ▁for ▁closing ▁its ▁shell