The shells that he is walking on in Proteus make Stephen think back to the shells that he pondered in Deasy's office and their resemblance to money: "Wild sea money." As the chapter goes on, he thinks more broadly of how shells express the protean flux of life.
Nestor associated shells with money because they are hollow, and an obsession with money empties life of its humanity. But the shells on the beach were once life forms. Houses on the shore are "Human shells," harboring life for a time and then emptied. Parts of the body, like Stephen's teeth, are constantly being built up or decayed into husks: "That one is going too. Shells. Ought I go to a dentist, I wonder, with that money?"
As a person walks on the beach, he or she is helping to break up the skeletons of countless small creatures: "Stephen closed his eyes to hear his boots crush crackling wrack and shells"; "Loose sand and shellgrit crusted her bare feet." The ocean is perpetually "Driving before it a loose drift of rubble, fanshoals of fishes, silly shells," working to crush these husks into bits.
Humanity is not exempt. One sentence after Stephen thinks of the surf pushing shells to shore, he imagines the drowned man's body rising to the surface and "bobbing landward." His body too will be recycled: "God becomes man becomes fish becomes barnacle goose becomes featherbed mountain. Dead breaths I living breathe, tread dead dust, devour a urinous offal from all dead."