Detail of the San Vitale Abel and Melchizedech Mosaic:
Basilica of San Vitale, Ravenna
The lion at the top is the symbol of St. Mark. The marsh and water birds at the bottom are also represented in the panel for St. Matthew on the left and the panels for Luke and John on the facing wall.
The Latin phrase in the book is secundum marcum, "according to Mark." In the Tridentine Mass the phrase appears when the priest or deacon reads a passage from Mark's Gospel.
Detail of the lion
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Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.