The altar was placed in the transept and was frequently surmounted by a baldachin composed of four or six columns supporting a light dome. Behind the altar in the centre of the apse was the throne (cathedra) occupied by the bishop (episcopus), being raised by steps from the semicircular stone seats (exedra) used by the presbyters, which were covered with carpets. The walls of the transept and apse were inlaid with mosaic inscriptions and pictures, in which the head of our Saviour, the figures of saints and holy emblems were the chief subjects. Deep blue, purple, and green were the prevailing colours and the letters were of gold. The floors were decorated with mosaic patterns. The roofs were either flat with sunken panels framed with mouldings and gilded ornaments, or else showed the open trussed wood-work, though the latter was the exception. Externally there was no attempt at enrichment, the exterior generally offering a great contrast to the lavish internal decorations. At the present day nearly all the basilicas have undergone transformation, the old roofs have been replaced, the walls covered with a modern adornment of pilasters and gaudy paintings, the colonnades have been broken through to allow of entrances to side-chapels, or disfigured by the heterogeneous decoration of the eighteenth century, and the exteriors treated with renaissance façades. Nevertheless the general plan and arrangements have remained substantially the same, and we have very interesting specimens of this class of building in St. Maria Maggiore, St. Agnese, San Clemente, and others, in Rome, San Appolinare, in Ravenna, the basilicas at Torcello, in the Venetian lagoons, and later examples in St. Ambrogio, of Milan, and St. Maria Sopra Minerva, in Rome. The basilica at Torcello was built mainly from fragments of an older church upon the mainland at Altino. The bishop’s throne is one of the most interesting and best preserved examples we have. The Greek name for this, cathedra, was the origin of our term cathedral, applied to churches containing the bishop’s seat, there being no architectural distinction between the buildings. From the tombs of the Romans the Christians derived their conception of the edifices which they used as baptisteries. Their exterior walls were either polygonal or circular, and of severe simplicity. The interiors were generally divided by a row of columns sustaining a round vault, and forming a circular enclosure in which the font was placed. A wall, carried on these columns, contained windows, and served as a lantern to light the building. This wall occasionally supported a dome. San Stephano Rotondo, in Rome; St. Angeli, in Perugia, and St. Vitale, in Ravenna, are the best examples among the many found in Italy.