▁the ▁United ▁States <0x0A> </s> ▁Oliv ella , ▁common ▁name ▁the ▁d war f ▁ol ives , ▁is ▁a ▁genus ▁of ▁small ▁pre dat ory ▁sea ▁sn ails , ▁marine ▁g ast rop od ▁m oll us cs ▁in ▁the ▁family ▁Oliv ell idae , ▁the ▁d war f ▁ol ives . ▁Oliv ella ▁is ▁the ▁type ▁genus ▁of ▁the ▁family . ▁ ▁Description ▁Ad ult s ▁in ▁the ▁Oliv ella ▁species ▁are ▁usually ▁quite ▁small , ▁hence ▁the ▁genus ▁has ▁the ▁common ▁name ▁" d war f ▁o live ". ▁Species ▁of ▁Ol iva ▁are ▁usually ▁larger , ▁but ▁there ▁are ▁exceptions . ▁ ▁The ▁shell ▁of ▁Oliv ella ▁usually ▁has ▁a ▁ke el - like ▁tw ist ▁at ▁the ▁anterior ▁end ▁of ▁the ▁col um ella . ▁The ▁wall ▁above ▁it ▁may ▁be ▁con ca ve ▁or ▁have ▁deep ▁fur rows . ▁The ▁inner ▁lip ▁can ▁sometimes ▁show ▁a ▁deep ▁call us , ▁and ▁in ▁many ▁cases ▁this ▁extends ▁over ▁the ▁par iet al ▁wall ▁to ▁the ▁end ▁of ▁the ▁a pert ure . ▁This ▁call us ▁formation ▁may ▁extend ▁to ▁the ▁sp ire ▁but ▁leave ▁the ▁s uture ▁open . ▁Most ▁species ▁of ▁Oliv ella ▁have ▁a ▁thin , ▁ch it in ous ▁oper cul um , ▁but ▁this ▁oper cul um ▁is ▁lack ing ▁in ▁Oliv ella ▁n ive a , ▁as ▁is ▁also ▁the ▁case ▁in ▁species ▁of ▁Ol iva . ▁ ▁Species ▁ ▁Species ▁in ▁the ▁genus ▁Oliv ella ▁include : ▁▁ ▁Oliv ella ▁act e oc