"At first, the work of making the Catacombs was done openly, without let or hindrance, by the Christians; the entrances to them were public on the high-road or on the hill-side, and the galleries and chambers were freely decorated with paintings of a sacred character. But early in the third century, it became necessary to withdraw them as much as possible from the public eye; new and often difficult entrances were now effected in the recesses of deserted _arenariæ_, and even the liberty of Christian art was cramped and fettered, lest what was holy should fall under the profane gaze of the unbaptized. "Each of these burial-places was called in ancient times either _hypogæum_, i. e. generically, a subterranean place, or _cœmeterium_, a sleeping-place, a new name of Christian origin which the pagans could only repeat, probably without understanding; sometimes also _martyrium_, or _confessio_ (its Latin equivalent), to signify that it was the burial-place of martyrs or confessors of the faith. An ordinary grave was called _locus_ or _loculus_, if it contained a single body; or _bisomum_, _trisomum_, or _quadrisomum_, if it contained two, three, or four. The graves were dug by _fossores_, and burial in them was called _depositio_. The galleries do not seem to have had any specific name; but the chambers were called _cubicula_. In most of these chambers, and sometimes also in the galleries themselves, one or more tombs are to be seen of a more elaborate kind; a long oblong _chasse_, like a sarcophagus, either hollowed out in the rock or built up of masonry, and closed by a heavy slab of marble lying horizontally on the top. The niche over tombs of this kind was of the same length as the grave, and generally vaulted in a semicircular form, whence they were called _arcosolia_. Sometimes, however, the niche retained the rectangular form, in which case there was no special name for it, but for distinction's sake we may be allowed to call it a table-tomb. Those of the _arcosolia_, which were also the tomb of martyrs, were used on the anniversaries of their deaths (_Natalitia_, or birthdays) as altars whereon the holy mysteries were celebrated; hence, whilst some of the _cubicula_ were only family-vaults, others were chapels, or places of public assembly. It is probable that the holy mysteries were celebrated also in the private vaults, on the anniversaries of the deaths of their occupants; and each one was sufficiently large in itself for use on these private occasions; but in order that as many as possible might assist at the public celebrations, two, three, or even four of the _cubicula_ were often made close together, all receiving light and air through one shaft or air-hole (_luminare_), pierced