The Transfiguration Mosaic at Sant'Apollinare in Classe
Basilica of Sant'Apollinare in Classe, Ravenna
This image is discussed in Margaret E. Frazier's "The Christian Realm:
Apse Themes,” in Weitzmann. The cross represents the transfigured Christ. To its left and right are Moses and Elijah, respectively. The figure below with the sheep is St. Apollinaris.
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Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.