Linda, The inner layer in the pectens is aragonite. Perhaps there is an organic layer connecting the two which is more easily dissolved. From: lindawbush <[log in to unmask]> To: [log in to unmask] Subject: Re: Shell composition from Re: crepidulas and mussels Date: Wed, 18 Dec 2002 07:44:16 -0500 Hi, All! In reference to the calcie/aragonite topic, I am wondering if the presence of calcite in some shells is responsible for their "misbehavior" in bleach, or if it is the presence of more organic matter in them. I have noticed that the inner layer of some Pectinids is very touchy to the beach and consequently watch them carefully. Other shells can stay indefinitely in bleach without damage. Comments? Cordially, Linda Allen Aigen [log in to unmask]