The medallion is part of the grand mosaic above the door to the chapel:
In the inner arc eight medallions portray virgin saints. They are not labeled. Each has the same embroidered collar and necklace of dark beads seen in the virgin medallions in the Euphrasian Basilica in Poreč and the Archiepiscopal Museum in Ravenna (both 6th century). The chief difference is that these virgins have crowns while in the earlier images they wear simple veils.
In the medallions left and right of the Madonna are two tonsured saints, the one on the left a young man dressed as a deacon and the other with white hair and beard.
Christ is in the top medallion of the outer arc, flanked by Saints Paul and Peter. The other Apostles are portrayed in the ten medallions on the left and right sides of the arc. The Apostle to the right of Peter is probably his brother Andrew, judging by the forked beard and untidy hair.
At the bottom ends of the outer arc are two images of Popes wearing papal tiaras. They appear to be of a later date than the rest of the mosaic. They are in much better condition, and the crown-like circlet at the base is not seen on any other image of a tiara until centuries later.1
Read more about portraits of Mary.
