The shell is an obvious symbol of baptism because of its association to water. In fact, a shell is often used to scoop up and sprinkle water during the baptismal ceremony and often designed as part of the decoration and symbolism of a baptismal font.
The shell also represents the journey of a Christian pilgrim. The journey to find salvation and rebirth of the soul. According to the Association for Gravestone Studies Website, Christians during the Middle Ages would venture out on their pilgrimage to the shrine of St. James of Compostella in Spain. After the pilgrimage they would wear a scallop shell as a sign that they had made the religious trek.