This singular feature, when taken in conjunction with the great tank or reservoir of the first story and the several smaller tanks or reservoirs discovered in the various corridors and sepulchral chambers--peculiarities and features possessed by no other catacomb--amply justifies the ancient appellation “ad Nymphas,” which no doubt exclusively belongs to the Cemetery of S. Priscilla, and which in several parts seems to preserve a memory of the baptisms of S. Peter. Over most of the catacombs--certainly over the more important--shortly after the Peace of the Church, basilicas or churches of various dimensions were erected for the accommodation of pilgrims and members of the Roman congregations who desired to visit and to venerate the sanctuaries of the subterranean cemeteries which soon became famous and objects of reverence in the Christian world. The basilica subsequently known as S. Sylvester, which was built over the Cemetery of Priscilla, no doubt before the year 336, has perhaps obtained a greater notoriety than any other of these fourth-century cemetery churches. Into this basilica, apparently shortly after its erection, were translated many of the bodies of martyrs, whose remains had been in the first instance deposited in the crypts of S. Priscilla beneath. The Pilgrim Itineraries dwell upon this, and especially mention how under the high altar of S. Sylvester two of the martyred sons of S. Felicitas rested--Felix and Philip. Into S. Sylvester, too, were brought the remains of the two martyr Popes, Marcellus and Marcellinus. There also Pope Sylvester, the builder of the basilica after whom it has been named, was interred; and at his feet Pope Siricius, the successor of Pope Damasus. Three more of the occupants of the papal dignity have been interred in this honoured sanctuary, namely, Liberius, A.D. 353-5; Celestinus, A.D. 422-32; and at a somewhat later date Pope Vigilius, A.D. 538-55; in all the remains of seven of the Bishops of Rome rested in S. Sylvester. Indeed, this little basilica ranks as the third of the sacred places of interment of the Bishops of Rome. The _first_ is on the Vatican Hill--in the immediate neighbourhood of the grave of S. Peter--where ten or eleven of the first occupants of the See of Rome lie. The _second_ is the famous so-called Papal Crypt in the Cemetery of S. Callistus on the Appian Way. The _third_ is the Basilica of S. Sylvester over the Cemetery of Priscilla. The _fourth_ is once more on the Vatican Hill, near the grave of S. Peter, in the stately church erected by the Emperor Constantine on the site of the little Memoria chapel of Linus. [Illustration: CHAPEL OF THE TOMBS OF THE III CENTURY BISHOPS OF ROME--PARTLY RESTORED--CATACOMB OF S. CALLISTUS]