▁above ▁the ▁surface ▁of ▁the ▁shell , ▁b ending ▁decided ly ▁back ward , ▁then ▁cur ving ▁and ▁nearly ▁meeting ▁in ▁front . ▁The ▁outer ▁lip ▁r ounds ▁gradually ▁to ▁near ▁the ▁anterior ▁end ▁of ▁the ▁a pert ure , ▁where ▁it ▁is ▁slightly ▁contract ed , ▁making ▁a ▁short , ▁broad ▁si ph onal ▁canal , ▁then ▁tw isting ▁ab rupt ly ▁back ward ▁produces ▁a ▁decided ▁not ch ▁before ▁joining ▁the ▁col um ella . ▁A ▁consp ic uous ▁var ix ▁is ▁formed ▁by ▁the ▁thick ening ▁of ▁the ▁last ▁rib , ▁beyond ▁which ▁the ▁outer ▁lip ▁is ▁thin , ▁white , ▁and ▁sharp - ed ged . ▁There ▁is ▁a ▁prominent , ▁to oth - like ▁projection ▁on ▁the ▁interior ▁of ▁the ▁a pert ure , ▁at ▁the ▁end ▁of ▁the ▁var ix , ▁more ▁consp ic uous ▁in ▁some ▁spec im ens ▁than ▁in ▁others , ▁and ▁seen ▁best ▁in ▁an ▁end ▁view . ▁The ▁interior ▁of ▁the ▁a pert ure ▁and ▁the ▁inner ▁lip ▁is ▁white . ▁The ▁col um ella ▁is ▁straight , ▁with ▁a ▁consp ic uous ▁layer ▁of ▁en am el ▁having ▁a ▁thin , ▁free ▁edge . ▁The ▁s uture ▁is ▁distinct , ▁und ulating . ▁The ▁subs ut ural ▁band ▁is ▁very ▁narrow , ▁con ca ve , ▁la pping ▁well ▁on ▁the ▁preceding ▁wh or l . ▁Prom inent , ▁strong , ▁ob lique , ▁rounded ▁rib s , ▁nine ▁on ▁the ▁body ▁wh or l , ▁separated ▁by ▁con ca ve