Pray remember the grotto. July 25 new style, and August 5 old style, is the day dedicated to St. James the Greater; and the correct thing to do in days of yore was to stick a shell in your hat or coat, and pay a visit on that day to the shrine of St. James of Compostella. Shell grottoes with an image of the saint were erected for the behoof of those who could not afford such pilgrimage, and the keeper of it reminded the passer-by to remember it was St. James's Day, and not to forget their offering to the saint.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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