The subterranean vaults of S. Peter's contain mere fragments of tombs, some precious as historical records, some valuable as works of art, swept together pell-mell from the ruins of the old basilica.
See the original letter to Ammanati, published from the Archivio Buonarroti, by Signor Milanesi, p. 535.
I am far from meaning that the earlier architects had not been guided by ancient authors. Alberti's Treatise on the Art of Building is a sufficient proof of their study of Vitruvius, and we know that Fabio Calvi translated that writer into Italian for Raphael. In the later Renaissance this study passed into purism.