Roughly, the Cemetery of Domitilla is composed of three distinct divisions or areas. The first, the work of the first and second centuries. In this area there are several famous historical centres, _e.g._ the tombs of the two saints Nereus and Achilles, and of the once famous S. Petronilla, and the well-known entrance or vestibule which opens on the Via Ardeatina, and the Chapel or Chamber of Ampliatus. The second area is the work of the third century, and the third dates from the last years of the third and the first quarter of the fourth. These areas have been connected with corridors of different periods in the second, third, and fourth centuries; the whole network is of very great extent. [Illustration: ENTRANCE TO THE CEMETERY OF S. DOMITILLA--(CRYPT OF THE FLAVIANS) CENTURY I] At the end of the sixth century, in the Pontificate of Siricius, great damage was occasioned to much of the earlier part of the cemetery by the construction of the Basilica of S. Petronilla, a building which also bears the names of Nereus and Achilles. The fame of these early martyrs and the number of pilgrims to their shrines in the closing years of the fourth century, induced Pope Siricius--regardless of the mischief which such a work would occasion to the many unknown graves of an early period--to build a somewhat large church or basilica over the tombs of SS. Nereus and Achilles and S. Petronilla. The position of the tombs of these two saints has been ascertained; the grave of Petronilla has also been localized with fair certainty. The high altar of the fourth-century basilica was placed over the graves of the two martyrs; the remains of Petronilla lay in a chamber behind the apse of the basilica; without, of course, maintaining the accuracy of the details of the sixth-century martyrology of Nereus and Achilles, the discoveries in the Cemetery of Domitilla have established the fact of the existence of these two traditional saints and martyrs. Scholars recognize now that much of the sixth-century martyrology was founded upon dependable tradition. The much disputed tradition of Petronilla, the wanderings of her body, and the question whether or not she was the daughter of S. Peter, is discussed in Appendix I., p. 277, where the story of her tomb is told at some length. The crypt of Ampliatus--another of the historic centres of this great catacomb, is situated in the middle of the area or district originally occupied by the tombs of the Christian members of the Imperial Flavian House. The decorations of the sepulchral chambers here and the style of inscriptions belong to the first century and first half of the second.