The 5th century Roman basilical plan has been well preserved inside. The edifice has a nave with side aisles. There are a total of forty-two Ionic columns supporting an entablature rather than arcades. Thirty-eight of the columns are of Hymettus marble quarried from near Athens, Greece, while the other four are in grey granite. The columns are spolia, and probably came from more than one ancient Roman building since they were not originally a matching set. They look uniform now because Fuga in the 18th century cut down in order to match them up. The 5th century figurative mosaic panels on the nave side walls are of first importance. The mosaics depicting Old Testament scenes, which makes them the oldest surviving cycle of Christian mosaic panels in a church in Rome.