▁the ▁aqu arium ▁trade ▁ ▁where ▁it ▁is ▁often ▁simply ▁referred ▁to ▁as ▁cro ce a . ▁ ▁Description ▁The ▁b oring ▁cl am ▁is ▁the ▁smallest ▁cl am ▁in ▁the ▁sub family ▁Tr id ac n inae ▁and ▁grows ▁to ▁a ▁maximum ▁shell ▁size ▁of ▁. ▁It ▁has ▁two , ▁thick ▁val ves ▁joined ▁together ▁by ▁a ▁h inge ▁which ▁is ▁typically ▁between ▁a ▁third ▁and ▁less ▁than ▁a ▁half ▁of ▁the ▁width ▁of ▁the ▁shell . ▁Typ ically ▁shell s ▁are ▁slightly ▁to ▁moder ately ▁el ong ate , ▁and ▁the ▁animal ▁is ▁strongly ▁infl ated , ▁especially ▁near ▁the ▁h inge . ▁The ▁upper ▁val ve ▁has ▁six ▁to ▁ten ▁fl att ish ▁fol ds ▁which ▁inter lock ▁at ▁the ▁margin ▁with ▁similar ▁fol ds ▁on ▁the ▁lower ▁val ve ▁so ▁that ▁they ▁can ▁cl amp ▁together ▁and ▁the ▁shell ▁can ▁close ▁tight ly . ▁The ▁lower ▁val ve ▁has ▁sl its ▁through ▁which ▁the ▁by ss al ▁threads ▁emer ge ▁which ▁secure ▁the ▁animal ▁to ▁the ▁se ab ed . ▁This ▁cl am ▁bur rows ▁into ▁the ▁substr ate ▁and ▁this ▁process ▁tends ▁to ▁wear ▁away ▁the ▁outer ▁surface ▁of ▁the ▁val ves ▁and ▁smooth ▁them ▁off . ▁They ▁also ▁may ▁become ▁dist orted ▁by ▁bur row ing ▁into ▁substr ates ▁of ▁une ven ▁hard ness . ▁ ▁The ▁colour ▁of ▁the ▁val ves ▁is ▁generally ▁grey ish - white , ▁sometimes ▁with ▁a ▁hint ▁of ▁p ink - or ange , ▁yellow ▁or ▁orange ▁colour ▁- this ▁colour