The seashell, especially the scallop, is a symbol of baptism in Christianity (the baptismal font is often shaped like a scallop or decorated with one, so is the dish used by priests to pour water over the heads of catechumens).
It is also a symbol associated with the apostle St James. Followers of St James way usually wear the scallop-shell on their hats or clothes. It is a symbol of journey or pilgrimage. Found on a grave, it may remind one that the person buried was a pilgrim, or represent their journey to a new life.
It may also be a symbol of fertility (Venus, the Roman goddess of fertility and love is often represented in ancient Renaissance art appearing from a scallop).
Journey; rebirth.