▁appearance . ▁ ▁They ▁are ▁grouped ▁in ▁the ▁same ▁super family ▁as ▁the ▁scal lo ps . ▁▁ ▁They ▁are ▁not ▁closely ▁related ▁to ▁true ▁o yst ers ▁( family ▁Ost re idae ); ▁however , ▁they ▁do ▁share ▁some ▁hab its ▁such ▁as ▁c ement ing ▁themselves ▁to ▁rocks ▁rather ▁than ▁att aching ▁themselves ▁by ▁a ▁by ss us . ▁ ▁The ▁two ▁hal ves ▁of ▁their ▁shell s ▁are ▁joined ▁with ▁a ▁ball - and - socket ▁type ▁of ▁h inge , ▁rather ▁than ▁with ▁a ▁to oth ed ▁h inge ▁as ▁is ▁more ▁common ▁in ▁other ▁b ival ves . ▁ ▁They ▁also ▁still ▁retain ▁v estig ial ▁anterior ▁and ▁posterior ▁aur icles ▁(" ears ", ▁tri angular ▁shell ▁f laps ) ▁along ▁the ▁h inge ▁line , ▁a ▁characteristic ▁feature ▁of ▁scal lo ps , ▁though ▁not ▁of ▁o yst ers . ▁ ▁As ▁is ▁the ▁case ▁in ▁all ▁scal lo ps , ▁Sp ond yl us ▁s pp . ▁have ▁multiple ▁eyes ▁around ▁the ▁edges ▁of ▁their ▁mant le , ▁and ▁they ▁have ▁relatively ▁well - develop ed ▁nerv ous ▁systems . ▁ ▁Their ▁nerv ous ▁gang lia ▁are ▁concentr ated ▁in ▁the ▁vis cer al ▁region , ▁with ▁recogn is able ▁opt ic ▁lo bes ▁connected ▁to ▁the ▁eyes . ▁ ▁E volution ary ▁history ▁The ▁genus ▁Sp ond yl us ▁appeared ▁in ▁the ▁Mes oz o ic ▁era , ▁and ▁is ▁known ▁in ▁the ▁foss il ▁records ▁from ▁the ▁Tri ass ic ▁Cass ian ▁b eds