Nor is there any greater resemblance in the excavations of the _arenaria_. These are large and lofty vaults, from sixteen to twenty feet wide, the arch of which often springs directly from the floor, so as to give the largest amount of sand with the least labour of excavation. The object was to remove as much material as possible; hence there was often only enough left to support the roof. The spacious passages of the _arenaria_ run in curved lines, avoiding sharp angles, so as to allow the free passage of the carts which carried away the excavated sand. In the Catacombs, on the contrary, as little material as possible was removed; hence the galleries are generally not more than three, or sometimes only two, feet wide, and run for the most part in straight lines, often crossing each other at quite acute angles, so that only very narrow carts can be used in cleaning out the accumulated _débris_ of centuries--a very tedious 41 process, which greatly increases the cost of exploration. The walls, moreover, are always vertical, and the roof sometimes quite flat, or only slightly arched. The wide difference in the principle of construction is obvious. The great object in the Catacombs has been to obtain the maximum of wall-surface, for the interment of the dead in the _loculi_ with which the galleries are lined throughout, with the minimum of excavation. The structural difference will at once be seen by comparing the irregular windings of the small _arenarium_ represented in the upper part of Figs. 3 and 26 with the straight and symmetrical galleries of the adjacent Catacomb. Connected with the Catacomb of St. Agnes is an extensive _arenarium_, whose spacious, grotto-like appearance is very different from that of the narrow sepulchral galleries beneath. In the floor of this _arenarium_ is a square shaft leading to the Catacomb, in which Dr. Northcote conjectures there was formerly a windlass for removing the excavated material. There are also footholes, for climbing the sides of the shaft, cut in the solid _tufa_, perhaps as a means of escape in the time of persecution. This _arenarium_, which was probably worked out and abandoned long before its connection with the Catacomb, may have been employed as a masked entrance to its crypts, when the more public one could not be safely used. Its spacious vaults may also have been a receptacle for the broken _tufa_ removed from the galleries beneath. [Illustration: Fig. 16.--Gallery in St. Hermes.] [Illustration: Fig. 17.--Part of Wall of Gallery in St. Hermes.]