The mosaics in the apse of the church of San Marcowere added during the reign of Pope Gregory IV (r. 827-44).
In the centre of the apse stands Christ, holding an open book in his left hand. It reads: EGO SVM LVX EGO SVM VITA EGO SVM RESVRRECTIO (I am the light. I am the life. I am the resurrection).
Apse Mosaic
To the left stands Pope Gregory IV (holding a model of the church), St Mark the Evangelist and St Felicissimus, while to the right stands Pope St Mark (r. 336), Pope St Agapetus I (r. 535-536) and St Agnes (holding a martyr’s crown). The hand of God the Father can be seen above the head of Christ, while under his feet we see a dove with a halo, perching on the edge of a fountain.
Pope Gregory IV & St Mark
In the centre of the lower band is the Agnus Dei (Lamb of God) flanked by twelve sheep (symbolising the twelve apostles). At either end of the sheep are the walled cities of, respectively, Bethlehem and Jerusalem.
At the bottom of the mosaic is a long inscription: VASTA THOLI PRIMO SISTVNT FVNDAMINE FVLCRA QVAE SALOMONICO FVLGENT SVB SIDERE SITV HAEC TIBI PROPQVE QVESTO PERFECIT PRAESVL HONORE GREGORII MARCE EXIMINIO CUM NOMINE QVRTVS TV QVOQVE POSCE DEVM VIVENDI TEMPORA DONET ET AD CAELI POST FVNVS SYDERA DVCAT (The enormous supports of the rotunda stand on a first-rate foundation, which shine situated beneath the Solomonic star. Gregory, the fourth of the name, leader, finished these for you and for your honour, famous Mark. You, therefore, ask God that he may grant him to live a long time and may lead him after death to the stars of heaven).
On the outside of the arch are roundels of Christ and symbols of the four Evangelists. The two full-length figures depict St Paul and St Peter.