The Apse in St. Paul Outside the Walls: Detail, St. James the Greater
The mosaic has two apostles labeled S[ANCTUS] IACOBVS ("Saint James"). I am assuming that the one with the grayer beard is St. James the Greater (Jacobus Major) as opposed to the other one, who would be James the Less (Jacobus Minor).
This St. James's segment of the Gloria in Excelsis reads D[OMI]NE FILI VNIGENITE I[ESUS] X[RISTUS], "Lord Jesus Christ, the only-begotten son." See the page for the main image for the context of this phrase.
More of St. James the Greater
Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.