Sarcophagus of Valentinian III: Left end of the lid
The relief in the upper section depicts a circular frame (not a wreath) with small "tails" sitting on a beehive shape. Within the frame is a staurogram with pendent alpha and omega. The sixteen circles inscribed below enclose crossed members that could represent the Cross itself or chariot wheels (c.f. Ezekiel 1).
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Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.