A few exceptions must be made to these remarks. To some cemeteries religious congregations of men or women had been attached from a very early date. These houses were established for the singing of the Divine praises day and night, in continuation of the primitive practice, when clergy and laity used to keep watch and sing psalms and hymns at the martyrs’ tombs. As long as this was done, the adjoining cemeteries remained at least partially accessible, and were preserved from utter oblivion. This is why we read of occasional visits to the Catacombs of St. Agnes, St. Cyriaca, St. Sebastian, and one or two others, even during the eleventh and twelfth centuries. Numerous inscriptions, which the making of the modern _Campo Santo_ in Rome has lately brought to light, demonstrate that there was a convent of nuns near San Lorenzo _fuori le mura_, as early as the fourth century. There had been another at the Basilica of St. Agnes since the days of Constantine; and Pope Sixtus III. established a congregation of men at the Basilica of St. Sebastian on the Via Appia. This last has an interesting connection with our subject; for the cemetery at St. Sebastian’s, which had been the temporary resting-place of the bodies of the Apostles Peter and Paul, was known as the Cemetery _ad Catacumbas_. How it came to be so called, or what is the meaning of the name, scholars are not agreed; neither would the settlement of the question add much to our knowledge of the Catacombs themselves, any more than if we could discover why another cemetery was called “The Cemetery at The Two Laurels” (_ad duas lauros_), and another, “The Cemetery at Cucumber Hill” (_ad divum cucumeris_). An accidental importance, however, attaches to the name of this particular cemetery, because it is now given also to all the other subterranean cemeteries of Rome, and even to the cemeteries of Naples, of Paris and Malta, of Sicily and Egypt, some of which have hardly any characteristics in common with them. CHAPTER IV. THEIR LOSS AND RECOVERY. From the middle of the ninth century till nearly the end of the sixteenth, the Roman Catacombs had no history, and were practically unknown. At various times indeed during the fifteenth century, a few friars and strangers from Scotland, from Sicily, and the North of Italy, had somehow found their way into a corner of the Catacomb of St. Callixtus; and a few men of learning also, about the same time, visited both this and some other cemeteries. All these visitors have left their names inscribed upon the walls; but neither the friars nor the archæologists seem to have recognised the value of what they saw, nor taken pains to communicate their discovery to others. The whole subject lay in complete darkness, when, in the year 1578, an accidental circumstance brought it to light.