Glory to Jesus Christ!
Welcome a’board the board LRowson!
Perhaps there is a legend I don’t know about that will explain it all. I have only some thoughts on the subject.
The best info I have is that the scallop shell was always used as a pouring implement. Sort of like a spoon without a long handle, they were easy to procure and commonly used for a lot of small jobs, like cutting soft things and spooning. Every kitchen would have some.
Scallops have been used as a symbol of pilgrimage. I can only conjecture about the reason: In pre-historic times people would walk great distances along beaches (and coastline bluff roads when the beaches gave way to rock). Even boats and ships wouldn’t leave sight of the shoreline and would find an anchorage by twilight, searching the beaches for food and water.
The connection between shells and water is a given, and Christians already had water as symbolic of life. As water will not have a definate shape (but is clear and nearly formless) in an illustration the shell may imply water.
Ancient people have used the scallop shell as a symbol of eternal life. Why, I can only guess, but it was regarded that way in pre-Christian times across cultures.
My guess that scallop shells were bought for use as pouring implements routinely in the ancient world and took on these other symbolic qualities later. Sometimes the shell is illustrated with three teardops representing the triple pourings or immersions in baptism.
Therefore my guess is that the shell came to represent the Baptism much like the cup (chalice or goblet) came to represent the Eucharist. Both symbols existed as real implements before they were associated with these Sacraments, and using ornate or well crafted versions will reinforce the early symbolism they had acquired.
Vestments are like that too, cloaks, robes and stoles existed before the Christian era. Now due to their symbolic qualities we expect our “men of the cloth” to wear them at our sacramental functions, even while we laypeople have gone on to other fashions.
