More than two centuries had now elapsed since the Catacombs were begun, and they had attained a degree of development far beyond anything that could have been anticipated. Divers modifications also had been gradually introduced in the execution of the work, and of these it will be worth while to give some account. We will take the cemetery of St. Callixtus as a specimen; for although in almost every Catacomb there is some difference in detail from the rest, caused either by some local peculiarity, or by the mere taste or caprice of those who made it, yet, in outline at least, the process of development must have been the same in all. It seems to have been something of this kind. At first a plot of ground in some suitable situation was either given by its Christian proprietor, or acquired by purchase, and secured by all necessary legal formalities—probably also enclosed by a wall or other fence—as a place of burial. A plan of excavation was then determined upon, and the work begun. Whilst as yet the _fossors_ had had no experience of the consistency of the material in which they were to labour, and whilst they imagined themselves to be providing for the burial only of a few, they would naturally work freely and without much regard to economy of space. But as time went on the necessity for economy would become apparent; then they had recourse to various devices for securing it. They lowered the floors both of the vaults and galleries, so that they might receive more tiers of graves. They made the galleries somewhat narrower than before, and closer to one another. At the angles of their intersection, where the friability of the rock would not admit of full-sized graves being cut, they made smaller graves for infants, that the space might not be wasted. They did the same also even in the shelves of rock which had been left as a necessary support between the several tiers of graves. Everywhere unnecessary labour was spared; no more soil was removed than was absolutely necessary for the purpose required. The graves were made wider at the head and shoulders, and narrower at the feet; and if two bodies were to be buried in a double grave (_locus bisomus_), the soil was excavated only in exact proportion, the feet of the one being generally laid by the head of the other.