Detail of the San Vitale Hospitality of Abraham:
The wreath below Jeremiah's name refers to his martyrdom in Egypt reported in Tertullian (Scorpiace, 8). The scroll represents his prophecies; the writing in it is represented by little marks that probably represent letters but could conceivably refer to some belief about his prophecies. He is represented along with Moses and Abraham as witnesses to God's covenant with his people, as explained in my page for the mosaic as a whole.
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Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.