and with an altarpiece The Burial of St Petronilla by Guercino (Giovanni Francesco Barbieri), 1623. South aisle The first chapel in the south aisle is the baptistry, commissioned by Pope Innocent XII and designed by Carlo Fontana, (great nephew of Domenico Fontana). The font was carved from the lid of the purple porphyry sarcophagus which had once held the remains of the Emperor Hadrian, and is surmounted with a gilt-bronze figure of the "Lamb of God". Fontana's reworked porphyry sarcophagus lid font replaced an earlier font, which was re-purposed from the sarcophagus of Probus, the fourth-century Prefect of Rome, and which was used for baptisms from the 15th century until the late 17th century, when Fontana's work was completed. Against the first pier of the aisle is the Monument to the Royal Stuarts: James Stuart, known as the "Old Pretender" and his sons, Charles Edward, known as "Bonnie Prince Charlie", and Henry, a cardinal. The tomb is a Neo-Classical design by Canova unveiled in 1819. Opposite it is the memorial of James Francis Edward Stuart's wife, Maria Clementina Sobieska. The second chapel is that of the Presentation of the Virgin and contains the memorials of Pope Benedict XV and Pope John XXIII. Against the piers are the tombs of Pope Pius X and Pope Innocent VIII. The large chapel off the south aisle is the Choir Chapel which contains the altar of the Immaculate Conception. At the entrance to the Sacristy is the tomb of Pope Pius VIII The south transept contains the altars of Saint Thomas, Saint Joseph and the Crucifixion of Saint Peter. The tomb of Fabio Chigi, Pope Alexander VII, towards the end of the aisle, is the work of Bernini and called by Lees-Milne "one of the greatest tombs of the Baroque Age". It occupies an awkward position, being set in a niche above a doorway into a small vestry, but Bernini has utilized the doorway in a symbolic manner. Pope Alexander kneels upon his tomb, facing outward. The tomb is supported on a large draped shroud in patterned red marble, and is supported by four female figures, of whom only the two at the front are fully visible. They represent Charity and Truth. The foot of Truth rests upon a globe of the world, her toe being pierced symbolically by the thorn of Protestant England. Coming forth, seemingly, from the doorway as if it were