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).
The scroll he carries reads DOMINE FILI UNIGENITE IESU CHRISTI, "Lord Jesus Christ, only-begotten son," a phrase from the hymn Gloria in Excelsis Deo. The other apostles in the mosaic hold scrolls with other phrases from the hymn.
Read more about St. James the Greater.
Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.