However this may be, the type of the early Christian church or basilica presented to view an elongated hall with two or four rows of pillars, dividing it into three or five aisles, with a lofty triumphal arch at the end of the central nave, leading into a wide open space raised some steps higher than the nave, and in which stood the altar. Beyond this was the invariable apse with its semi-domed ceiling adorned with mosaics, and containing, elevated by a few steps above the floor, the throne of the Bishop, and the seats of the Presbyters. The whole building was covered with an open wooden roof. Some of these early churches have been preserved or restored in Rome--such as San Paolo fuori le Mura, Sta Maria Maggiore and San Clemente. There is every reason to believe that the above was the usual form of early churches in the West. At Ravenna, which was the principal city in Italy during the Lower Empire, being the seat of the Exarch, the representative of the Emperor in the West, there are fine examples of the various kinds of early Christian religious edifices, dating from the fifth to the seventh century. The great Church or Basilica, used for the assembly of the whole congregation, is represented in St Apollinare Nuovo. It has the usual row of columns on either side of the nave, separating it from the side aisles, and supporting a flat upper wall splendidly decorated with mosaics, the whole being ornamented with Roman details. The upper portion of the wall is pierced with clerestory windows, and at the east end is the great apse. The Baptistery or Ceremonial Church is as usual octagonal and is domed. Here also the walls are covered with fine mosaics. Another extremely interesting building at Ravenna is the church of San Vitale. This edifice (whether designed as a monument or as a church is uncertain) is octagonal and domed, very much after the style of the temple of Minerva Medica and similar Roman structures. San Vitale has a special interest from its having formed the model adopted by Charlemagne for the church which he erected at Aix-la-Chapelle, to serve also as his own mausoleum. It thus constitutes an example of a Roman design reproduced in Ravenna, under the late Empire, as a Christian structure, and again serving as a model for a mediæval mausoleum as late as the eighth century. This shows distinctly the continuity of Roman design and its direct influence on the art of later times.