My friend John, in a former life my Deputy Principal at The City Lit, wrote to point out that the shell associated with St James of Compostela is the scallop and not the oyster. He should certainly know having himself done part of the pilgrimage route to Compostela. He is, of course, correct. He always was! But here in England the association of St James, or at least his saint's day, with oysters is also well established. But, yet again, when I say 'well established' I am unable to put a date on that, earlier than the early 19th century; and certainly in medieval catholic England the association with scallops would have been well known. Folklore can be so frustrating at times.
It is worth noting that the scallop shell has also become a badge of Christian pilgrimage in general, although first associated with St James. There are many different legends linking St James and the scallop shell. My favourite says that after James' martyrdom in Jerusalem, his body was brought to Spain, but the ship carrying the body was wrecked off the coast near Compostela, and when the body washed ashore it was found to be covered in scallop shells. Cynics might say a good story with which to sell pilgrims cheap scallop shell souvenirs! I really couldn't comment.
Thanks, John, for helping to throw further light on the story of Grottos.