A chapel on the right of the church contains the monument of Cardinal Vulcani, 1322, supporting his figure, with Faith, Hope, and Charity sculptured in high relief below. Near the door is that of Cardinal Adimari, 1432, who died here after an ineffectual mission to the anti-pope Pedro da' Luna. In the left transept was a fine Perugino (removed 1867); in the right transept is the tomb of Pope Gregory XI., by Pietro Paolo Olivieri, erected by the senate in gratitude for his having restored the papal court to Rome from Avignon. A bas-relief represents his triumphal entry, with St. Catherine of Siena, by whose entreaties he was induced to return, walking before his mule. A breach in the walls indicates the ruinous state into which Rome had fallen, the chair of St. Peter is represented as floating back through the air, while an angel carries the papal tiara and keys; a metaphorical figure of Rome is coming forth to welcome the pope.
"The greatest part of the praise due to Gregory's return to Rome belongs to St. Catherine of Siena, who, with infinite courage, travelled to Avignon, and persuaded the pope to return, and by his presence to dispel the evils which disgraced Italy, in consequence of the absence of the popes. Thus it is not to be wondered at, that those writers, who rightly understand the matter, should have said that Catherine, the virgin of Siena, brought back to God the abandoned apostolical chair upon her shoulders."--_Ughelli, Ital. Sacra_, vi. col. 45.
Near Pope Gregory's tomb some blackened marks in the wall are shown as holes made by the (gigantic) knees of St. Peter, when he knelt to pray that Simon Magus might be dropped by the demons he had invoked to support him in the air, which he is said to have done to show his power on this spot.
"When the error of Simon was spreading farther and farther, the illustrious pair of men, Peter and Paul, the rulers of the Church, arrested it by going thither, who suddenly exhibited as dead, Simon, the putative God, on his appearance. For when Simon declared that he would ascend aloft into heaven, the servants of God cast him headlong to the earth, and though this occurrence was wonderful in itself, it was not wonderful under the circumstances, for it was Peter who did it, he who bears with him the keys of heaven, ... it was Paul who did it, he who was caught up into the third heaven."--_St. Cyril of Jerusalem._