Lutherans: Shells, baptism, why?

Since I am almost certain someone will come in and think the title of this question needs editing, I'm going to request that you leave it as it stands. ### The full/expanded question: I have seen shells used in Lutheran imagery in relation to infant baptism. What is the Scriptural basis for this practice of using a shell? If knowledgeable -- additionally, what is the historical basis for Lutherans to do this in their denominational history? I realize there other Catholic spinoff denominations may use the same imagery, but for the scope of this question I'm only asking about the Lutherans.