hierarchy of plans
There is the degree of hierarchy of the two three-sided series of sepulchers--Sepulchra adjacent the Porticus Neronianae and Sepulchra Libertorum et Servorum--where one is a more advanced/developed form of the other. These are closely related to the repetition of the Gymnasium but they also introduce a linear motif that has not yet been addressed. The linear repetition motif is also very evident in various porticus plans.
Sep. Mariae Honorij Congiig.
The Sepulchrum Mariae Honorij Congiig., the tomb of Empress Maria the first wife of Honorius, is the second to last building of ancient Rome designated within the Ichnographia Campus Martius. Piranesi mistakenly represents the tomb of Maria as separate from the tomb of Honorius; Maria's death may well have been the reason Honorius erected his family mausoleum around 400. Despite Piranesi's incorrectness, the position of Maria's tomb on the large plan falls within the area of the Vatican Hill that is actually occupied by St. Peter's Basilica, thus adding some veracity to the sepulcher's location. Moreover, Maria's tomb is within a courtyard surrounded on three sides by what appear to be other sepulchers whose placement within the Ichnographia partially coincides with the necropolis still in existence under St. Peter's Basilica, which is the traditionally held burial place of St. Peter himself. Another interesting detail is Piranesi's placement of Maria's tomb directly in front of the Templum Apollonis that is attached to the Circus of Caligula and Nero. According to "The Life of Sylvester" within the Liber Pontificalis, the original St. Peter's Basilica was built "in the shrine of Apollo," thus indicating that Piranesi's placement of the sepulcher of Maria within the Ichnographia was not altogether incorrect, but rather subtly and cleverly making note of the ancient Rome's pagan to Christian inversion.