Detail of the Hospitality of Abraham Mosaic:
Basilica of San Vitale, Ravenna, Italy
The ox at the top is the symbol of St. Luke. The Latin phrase in the book is secundum lucam, "according to Luke." In the Tridentine Mass the phrase appears when the priest or deacon reads a passage from Luke's Gospel.
The saint's pallium and the basket of scrolls are conventional attributes of philosophers (Jensen, Face to Face, 44). The water bird in the marsh at the bottom parallels the ducks in the corresponding panel with St. John on the left and the panels for Mark and Matthew on the facing wall.
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Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.