The one point in which an essential difference occurs is the position of the altar, or communion table, and that of the Bishop's chair, or throne. In the classic basilica the apse was the tribunal, and a raised seat with a tesselated pavement occupied the central position in it, and was the justice-seat of the presiding judge; and in the sweep of the apse, seats right and left, at a lower elevation, were provided for assessors or assistant-judges. In front of the president was placed a small altar. The whole of these arrangements were copied in the basilica churches. The seat of the president became the bishop's throne, the seats for assessors were appropriated to the clergy, and the altar retained substantially its old position in front of the apse, generally with a canopy erected over it. This disposition continues in basilica churches to the present day. At St. Peter's in Rome, for example, the Pope occupies a throne in the middle of the apse, and says mass with his face turned towards the congregation at the high altar, which stands in front of his throne under a vast baldacchino or canopy; but in Western Christendom generally a change has been made,--the altar has been placed in the apse where the bishop's throne formerly stood, and the throne of the bishop and stalls of his clergy have been displaced, and are to be found at the sides of the choir or presbytery. [Illustration: FIG. 157.--SANT' APOLLINARE, RAVENNA. PART OF THE ARCADE AND APSE.] Many basilica churches were erected out of fragments taken from older buildings, and present a curious mixture of columns, capitals, &c.; others, especially those at Ravenna, exhibit more care, and are noble specimens of ancient and severe architectural work. The illustration which we give of part of the nave, arcade, and apse of one of these, Sant' Apollinare in Classe, shows the dignified yet ornate aspect of one of the most carefully executed of these buildings (Fig. 157). In some of these churches the decorations are chiefly in mosaic, and are extremely striking. Our illustration of the apse of the great basilica of St. Paul without the walls (Fig. 158) may be taken as a fair specimen of the general arrangement and treatment of the crowd of sacred figures and subjects which it is customary to represent in these situations; but it can of course convey no idea of the brilliant effect produced by powerful colouring executed in mosaic, the most luminous of all methods of enrichment. The floor of most of them was formed in the style of mosaic known as "opus Alexandrinum," and the large sweeping, curved bands of coloured material with which the main outlines of the patterns are defined, and the general harmony of colour among the porphyries and other hard stones with which these pavements were executed, combine to satisfy the eye. A splendid specimen of opus Alexandrinum, the finest north of the Alps, exists in the presbytery of Westminster Abbey.