As Mr. Deasy counts out Stephen's wages in Nestor, "Stephen's embarrassed hand moved over the shells heaped in the cold stone mortar: whelks and money cowries and leopard shells: and this, whorled as an emir's turban, and this, the scallop of saint James. An old pilgrim's hoard, dead treasure, hollow shells." These are common shells that could be collected while walking on many different beaches, but the scallop shell is symbolically associated with St. James the Greater, whose shrine at Compostela, Spain, has drawn religious pilgrims from the Middle Ages to the present—hence Stephen's fancy that the mortar full of shells is "An old pilgrim's hoard."
Compostela lies near the sea, in Galicia, and pilgrims apparently got in the habit of bringing a shell with them as they returned inland. "Being light and small in size, the scallop shell was ideal for gathering water from a spring or improvising tableware at the side of the path" (www.mscvocations.com). In time, pilgrims who had completed the journey got in the habit of wearing a scallop shell on their clothing as a badge, symbolizing their spiritual accomplishment. It appears that Mr. Deasy too is in the habit of walking along the shore in his own seaside village, collecting beautiful shells.
But for this materialistic pilgrim the shells do not (Stephen thinks) symbolize religious observance. They are a "hoard, dead treasure, hollow shells," their hollowness suggesting (in part, perhaps, because of the expression "money cowries") the emptiness of the old man's obsession with money. Money is no less symbolic than scallop shells are, and Stephen thinks of the payment being handed to him too as "hollow shells. Symbols too of beauty and of power. A lump in my pocket: symbols soiled by greed and misery." Possibly the reader should also make a connection between this "dead treasure" and the image of Sargent as a snail about to be crushed under the world's tramping feet, for if Mr. Deasy is an old militarist sending young men out into the fray of battle, then he makes "hollow shells" of human lives as well.
Stephen's meditations on shells continue in Proteus, beginning with the Deasy-connected fancy that they are "Wild sea money."