▁part ▁of ▁the ▁C ret ace ous ▁and ▁are ▁known ▁as ▁heter omorph s . ▁ ▁Perhaps ▁the ▁most ▁extreme ▁and ▁b izar re - looking ▁example ▁of ▁a ▁heter omorph ▁is ▁N ipp on ites , ▁which ▁appears ▁to ▁be ▁a ▁t angle ▁of ▁irregular ▁wh or ls ▁lack ing ▁any ▁obvious ▁symmetric ▁co iling . ▁Upon ▁closer ▁ins pection , ▁though , ▁the ▁shell ▁proves ▁to ▁be ▁a ▁three - dimensional ▁network ▁of ▁connected ▁" U " ▁shapes . ▁ ▁occurs ▁in ▁rocks ▁of ▁the ▁upper ▁part ▁of ▁the ▁C ret ace ous ▁in ▁Japan ▁and ▁the ▁United ▁States . ▁ ▁Am mon ites ▁vary ▁greatly ▁in ▁the ▁orn ament ation ▁( sur face ▁relief ) ▁of ▁their ▁shell s . ▁Some ▁may ▁be ▁smooth ▁and ▁relatively ▁feature less , ▁except ▁for ▁growth ▁lines , ▁and ▁res emble ▁that ▁of ▁the ▁modern ▁N aut ilus . ▁In ▁others , ▁various ▁patterns ▁of ▁spir al ▁rid ges ▁and ▁rib s ▁or ▁even ▁sp ines ▁are ▁shown . ▁This ▁type ▁of ▁orn ament ation ▁of ▁the ▁shell ▁is ▁especially ▁evident ▁in ▁the ▁later ▁am mon ites ▁of ▁the ▁C ret ace ous . ▁ ▁A pty chus ▁▁ ▁Some ▁am mon ites ▁have ▁been ▁found ▁in ▁association ▁with ▁a ▁single ▁horn y ▁plate ▁or ▁a ▁pair ▁of ▁cal cit ic ▁pl ates . ▁In ▁the ▁past , ▁these ▁pl ates ▁were ▁assumed ▁to ▁serve ▁in ▁closing ▁the ▁opening ▁of ▁the ▁shell ▁in ▁much ▁the ▁same ▁way ▁as ▁an ▁oper cul um , ▁but