Why a shell?
Legend has is that while St. Augustine was endeavoring to write a treatise on the Holy Trinity he saw a small boy using a seashell to carry water from the ocean to a small hole on the beach. When asked what he was trying to do the boy said he was trying to put the whole ocean into the hole, which Augustine observed was impossible. Then the boy said it was no more impossible than what Augustine was attempting to do. The boy disappeared and many believe it was the Christ Child himself there to give Augustine a kind lesson in the smallness of human understanding in the face of holy mystery.