The casual visitor cannot, of course, expect to be able to distinguish the limits of the several areæ which he traverses in his hurried subterranean walk; nevertheless, if he keeps his eyes open, he cannot fail to recognise occasional tokens of the transition; as, for instance, when he finds himself passing from a higher to a lower level, or _vice versâ_, or when the path which he is pursuing leads him through a wall of broken graves, so that it has been necessary perhaps to strengthen the points of connection by masonry. Any one who desires to study this branch of the subject, will find it fully treated of, and made easily intelligible, by numerous plans and illustrations either in the original work of De Rossi, or in the English abridgment of it. The present popular manual proposes to itself a more humble task. We propose to describe the principal objects of interest which are shown to strangers, and to supply such historical or archæological information as will give them a greater interest in, and a keener appreciation of, the importance of what they see. [Illustration: _Entrance to the Cemetery of St. Callixtus._] Without further preface, then, let us set out on our walk. Let us proceed along the Via Appia till we come to a doorway on the right-hand side, over which we read the words “Cœmeterium S. Callixti.” On entering the vineyard our attention is first arrested by a ruined monument standing close beside us. We shall have already seen others more or less like it on both sides of the road since we came out of the city; and in answer to our inquiries we shall have learnt that they are the remains of what were once grand Pagan tombs, covered, probably, with marble and ornamented with sculpture. Without stopping then to inquire whether anything special is known about the history of this particular mausoleum, we will walk forward to another more modest building standing in the middle of the vineyard. It looks small and mean; and if we could enter it, we should find that it is used only as a convenient magazine for the stowing away of fragments of sarcophagi, or tombstones, extracted from the cemetery which underlies it. Yet its apsidal termination, and the other apses on either side of the building, naturally suggest to us that it must once have had something of an ecclesiastical character. In truth, it was one of the “numerous buildings constructed throughout the cemeteries” by Fabian, pope and martyr, in the middle of the third century (see page 27), and was known to ancient pilgrims as the _cella memoriæ_, or chapel of St. Sixtus and of St. Cæcilia, being built immediately over the tombs of those martyrs. Originally, the end or fourth side of the building was unenclosed, that so larger numbers of the faithful might assist at the celebration of the holy mysteries: indeed the side-walls themselves were not at first continued to their present length, but the building consisted of little else than the three _exedræ_ or apses.