If we examine the ground round this ancient chapel, we shall see that it was once used as a place of burial. All round it, but not within it, nor yet quite close to its walls, but just at a sufficient distance from them to allow space for the channels which carried off the water from the roof (traces of which channels still remain), deep graves are dug, of various sizes, but all arranged according to a regular plan of orientation. These graves are made chiefly with blocks of tufa; but bricks also are used, and thick layers of mortar. Some of them were cased inside with marble, or at least had slabs of that material at the top and bottom, the upper surface of the one serving as the bottom of another; and it is worthy of remark that on several of these slabs were inscribed epitaphs of the usual kind, though their position would necessarily conceal them from every human eye. Some of the graves were made of sufficient depth to receive ten bodies, one over the other; some could only receive four; and occasionally only a single sarcophagus occupied the grave. The average, however, may be taken at four bodies in each grave, which would give a total of eight thousand persons buried over the first area of the Catacomb of St. Callixtus. Of course this cemetery, being in the open air, is no part of the catacomb—properly so called; it is of later date, having been made in the fourth and fifth centuries, but it observes precisely the same limits as the catacomb. The boundary-wall may still be seen at no great distance from the chapel; and beyond it no graves are to be found either in the cemetery or in the catacomb; a fact which shows with what care the rights of private property were respected below ground as well as above.