Sea shells are classified as animals by Customs. I know, I know....but that is the law. And trust me, they may ask you. It happened to me last year while leaving the Freedom Of The Seas. We had some shells and we were required to retrieve them from our baggage and surrender them before leaving the customs area.
In their defense, Customs is trying to keep diseases out of the U.S. And there is a possibility, however small, that even shells from dead sea creatures could harbor something. Google it.