▁and ▁are ▁to ▁varying ▁degrees ▁convex ▁when ▁seen ▁from ▁the ▁front , ▁distingu ishing ▁them ▁from ▁na util oid ▁sept a ▁which ▁are ▁typically ▁simple ▁con ca ve ▁d ish - sh aped ▁structures . ▁The ▁topology ▁of ▁the ▁sept a , ▁especially ▁around ▁the ▁rim , ▁results ▁in ▁the ▁various ▁s uture ▁patterns ▁found . ▁ ▁S uture ▁patterns ▁▁ ▁While ▁nearly ▁all ▁na util oid s ▁show ▁g ently ▁cur ving ▁s ut ures , ▁the ▁am m ono id ▁s uture ▁line ▁( the ▁intersection ▁of ▁the ▁sept um ▁with ▁the ▁outer ▁shell ) ▁is ▁vari ably ▁fol ded , ▁forming ▁s add les ▁(" pe aks " ▁which ▁point ▁towards ▁the ▁a pert ure ) ▁and ▁lo bes ▁(" val le ys " ▁which ▁point ▁away ▁from ▁the ▁a pert ure ). ▁The ▁s uture ▁line ▁has ▁four ▁main ▁regions . ▁The ▁external ▁or ▁vent ral ▁region ▁refers ▁to ▁s ut ures ▁along ▁the ▁lower ▁( outer ) ▁edge ▁of ▁the ▁shell , ▁where ▁the ▁left ▁and ▁right ▁s uture ▁lines ▁meet . ▁The ▁external ▁s addle ▁lies ▁directly ▁on ▁the ▁lower ▁mid line ▁of ▁the ▁shell ▁and ▁is ▁ed ged ▁by ▁external ▁lo bes . ▁On ▁s uture ▁diag rams ▁the ▁external ▁s addle ▁is ▁supplied ▁with ▁an ▁arrow ▁which ▁typically ▁points ▁towards ▁the ▁a pert ure . ▁The ▁later al ▁region ▁involves ▁the ▁first ▁s addle ▁and ▁lo be ▁pair ▁past ▁the ▁external ▁region ▁as ▁the ▁s uture ▁line ▁extends ▁up ▁the ▁side ▁of ▁the ▁shell .