The examples in Rome of circular or polygonal buildings are the Baptistry which forms part of the group of buildings of =S. John Lateran=, the =Tomb of S. Constanza=, the daughter of Constantine, which was converted into a church in 1256, and the church of =S. Stefano Rotondo=. The general character of the Roman tomb was a circular mass, superimposed on a square podium. The cylindrical mass was sometimes decorated with pilasters, supporting an entablature, and occasionally was surrounded by a peristyle, while its roof was apt to be conical. In early Christian architecture this principle of construction was developed. The peristyle was enclosed by outer walls, and the lower part of the walls of the cylindrical mass was replaced by columns. Thus, in the =Baptistry of S. John=, which has been called the =Baptistry of Constantine=, the conical roof is supported by a circle of eight columns, in two stories. The =Tomb of S. Constanza= has a dome which is supported on twelve pairs of granite columns, while the wall of the circular aisle is inset with sixteen recesses, alternately apsidal and rectangular in shape, one of the latter being opened through to form the entrance. The sarcophagus of the saint which formerly occupied one of the niches, is now in the Vatican Museum. Its sides are carved with genii gathering grapes--a motive which is also represented in the mosaics that adorn the vaulting of the church’s circular aisle. =S. Stefano Rotondo=, though much reduced from its original size, is said to be still the largest circular church in existence. The wall of the cylinder, surmounted by a wooden conical roof, is supported on a circular entablature, carried by antique columns. It was surrounded, when built by Simplicius in the fifth century, by double circular aisles, covered by a sloping roof. The latter was supported by columns and arches, while the external wall was decorated with pilasters. Traces of these are still apparent; otherwise the outer aisle has disappeared and the present exterior represents the walling up of the spaces between the columns. This was done by Nicholas V in the fifteenth century, by which time the edifice, once richly decorated with marble veneers and mosaics, had fallen into decay. Its lateral walls are now covered with horribly naturalistic scenes of martyrdom, executed at the end of the seventeenth century.