Many of these _arenaria_ may be observed excavated in the hill-sides near Rome; but except when incidentally forming part of a Catacomb, they have never been found to contain a single grave. Indeed, in consequence of the utter unfitness of the _pozzolana_ for the purposes of Christian sepulture, the intrusion of a deposit 42 of that material into the area of a Catacomb prevented the extension or necessitated the diversion of its galleries. Moreover, where the attempt has been made to convert an _arenarium_ into a Christian cemetery, the changes which have been made show conclusively its original unfitness for the latter purpose. The accompanying section of a gallery in the Catacomb of St. Hermes will exhibit the structural additions necessary to adopt an _arenarium_ for Christian sepulture. The sides of the semi-eliptical vault had to be built up with brick-work, leaving only a narrow passage in the middle. The _loculi_ were spaces left in the masonry, in which the mouldering skeletons may still be seen. The openings were closed with slabs in the usual manner, as shown in the elevation, (Fig. 17,) except at the top, where they cover the grave obliquely, like the roof of a house. The vault is often arched with brick-work, and at the intersection of the galleries 43 has sometimes to be supported by a solid pier of masonry. In part of an ancient _arenarium_ converted into a cemetery in the Catacomb of St. Priscilla similar constructions may be seen. The long walls and numerous pillars of brick-work concealing and sustaining the _tufa_, and the irregular windings of the passages, show at once the vast difference between the _arenarium_ and the Catacomb, and the immense labour and expense required to convert the former into the latter.