Detail of the San Vitale Hospitality of Abraham:
St. John the Evangelist
Basilica of San Vitale, Ravenna
This is in the upper left corner of the mosaic; St. Luke's portrait is in the upper right. In corresponding positions on the facing wall are portraits of St. Matthew and St. Mark.
The saint's pallium and the basket of scrolls are conventional attributes of philosophers (Jensen, 44). The Latin phrase in the book is secundum iohannem, "according to John." In the Tridentine Mass the phrase appears when the priest or deacon reads a passage from John's Gospel.
The eagle is the symbol of St. John.
I have not yet been able to explain the ducks swimming in the marsh at the bottom of the composition. "Birds of the air" are referenced dozens of times in the Bible, but ducks or water fowl in general are not. The corresponding panel with St. Luke on the right also has a marsh at the bottom, with a water bird, and the same is true of the evangelists on the facing wall.
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Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.