passage.
12. Passages destitute of tombs.
13. Narrow apertures between adjoining galleries.
14-17. Arcosolia.
18-32. Cubicula.
33. Chapel with vestibule and apse, and two chairs.
34. Double chapel with three chairs.
35. Large chapel in five divisions.]

In complete agreement with Jerome's vivid picture the visitor to the
Roman Catacombs finds himself in a vast labyrinth of narrow galleries,
usually from 3 to 4 ft. in width, interspersed with small chambers, all
excavated at successive levels, in the strata of volcanic rock subjacent
to the city and its environs, and constructed originally for the
interment of the Christian dead. The galleries are not the way of access
to the cemeteries, but are themselves the cemeteries, the dead being
buried in long low horizontal recesses, excavated in the vertical walls
of the passages, rising tier above tier like the berths in a ship, from
a few inches above the floor to the springing of the arched ceiling, to
the number of five, six or even sometimes twelve ranges. These galleries
are not arranged on any definite plan, but, as will be seen from the
plan (fig. 1), they intersect one another at different angles, producing
an intricate network which it is almost impossible to reduce to any
system. They generally run in straight lines, and as a rule preserve the
same level. The different storeys of galleries lie one below the other
(fig. 2) to the number of four or five (in one part of the cemetery of
St Calixtus they reach seven storeys), and communicate with one another
by stairs cut out of the living rock. Light and air are introduced by
means of vertical shafts (_luminaria_) running up to the outer air, and
often serving for several storeys. The drawing (fig. 3) from Northcote
gives a very correct idea of these galleries, with the tiers of graves
pierced in the walls. The doorways which are seen interrupting the lines
of graves are those of the family sepulchral chambers, or _cubicula_, of
which we shall speak more particularly hereafter.

The graves, or _loculi_, as they are commonly designated, were, in the
Christian cemeteries, with only a few exceptions (Padre Marchi produces
some from the cemetery of St Ciriaca, _Monum. primitiv._ tav. xiv.
xliii. xliv.), parallel with the length of the gallery. In the pagan
cemeteries, on the other hand, the sepulchral recess as a rule entered
the rock like an oven at right angles to the corridor, the body bein