A peculiar form of grave common in these chambers, as well as in the galleries, is that known as the _arcosolium_, or arched tomb. It consists of a recess in the wall, having a grave, often double or triple, excavated in the _tufa_, or built with masonry, like a solid sarcophagus, and closed with a marble slab. These are seen in the plan, Fig. 9, in the section, Fig. 10, at G and E in Fig. 15, and in perspective in Figs. 11 and 12. Sometimes the recess is rectangular instead of arched, and is then called by De Rossi _sepolcro a mensa_, or table tomb. Sometimes the arch was segmental, especially when constructed of masonry.[17] An example of both sorts is seen in the accompanying engraving of a _cubiculum_ in the Catacomb of St. Prætextatus. The narrow door into the corridor is also seen, and the stucco capitals and bases of the columns. In course of time these _arcosolia_ were used as altars for the celebration of the eucharist, 31 and eventually grave abuses arose from the superstitious veneration paid to the relics of the martyr or confessor interred therein. Frequently, also, the back of this arched recess was pierced with graves of a later date, often directly through a painting,[18] in order to obtain a resting place near the bodies of the saints. [Illustration: Fig. 14.--Section of the Catacomb of Callixtus.] Hitherto only one level of the Catacombs has been described, but frequently “beneath this depth there is a lower deep,” or even three or four tiers of galleries, excavated as the upper ones became filled with graves. Thus there are sometimes as many as five stories, or _piani_, as they are called, one beneath the other. These are carefully maintained horizontal, to avoid breaking through the floor of the one above or the roof of the one below, the danger of which would be very great if the strict level were departed from. For the same reason the different _piani_ were generally separated by a thick stratum of solid _tufa_. The relative position of these levels is shown by the following engraving, reduced from De Rossi. It represents a section of the Crypt of St. Lucina, a part of the Cemetery of Callixtus. The dark colored stratum, marked I in the margin, is entirely made up of the _débris_ of ancient monuments, buildings, and other materials accumulated in the course of ages in this place to the depth of eight feet. It has completely buried the ancient roads, except where excavated, as shown in the engraving. The next stratum, II, is of solid grayish _tufa_. In this the first level or _piano_, φ, is excavated. It is not more than twenty feet below the surface, and in many places only half that depth. Consequently its area is comparatively limited, because if extended it would have run out into the open air, from the sloping of the ground in which it is 32 dug. The next stratum, III, is softer and more easily worked, and therefore is that in which are found the most important and extensive _piani_ of galleries.