The ceiling features the four Evangelists with Christ in the center, as well as four angels above the four doctors of the Western Church: Saints Ambrose, Jerome, Augustine and Gregory.
The apse decorations were all designed by Francis Cranmer Penrose and the mosaics were manufactured by Salviati.
The twelve trefoil niches contain the Apostles (St. Paul having replaced St. Simon), surrounding a resurrected Christ and five angels.
Holy Cross Palermo
Barr, Sheldon. Venetian Glass Mosaics: 1860-1917. London: Antique Collectors' Club, 2008. 127.