Sec. 5. The sixth century basilicas of Ravenna, Sant' Apollinare in Classe and Sant' Apollinare Nuovo, differ in plan from the Roman basilicas (1) in the fact that they have always had the altar at the east, and the entrance at the west end; (2) by substituting, for a colonnaded atrium, a closed porch or _narthex_ in front of the entrance of the building. In process of time, two of the greater Roman basilicas, San Paolo and San Lorenzo fuori le Mura, were enlarged in a westward direction, so that the positions of the altar and entrance were reversed; and, in several of the early basilicas at Rome, a space near the entrance of the nave was screened off, from which penitents and catechumens might watch the service. But, in the first instance, the eastern chancel and the structural _narthex_ appear to have been introduced from the eastern empire. Neither at Ravenna nor at Rome did bell-towers originally form part of the plan of the basilica: the round _campanili_ of both churches at Ravenna are certainly later additions. It may also be noted (1) that ordinarily the aisles were single, not double as at old St Peter's. (2) The columned screen of the apse at old St Peter's appears to have been exceptional. The ordinary screen or _cancelli_, from which is derived our word "chancel" for the space thus enclosed, was a low wall. This is the arrangement at the basilica of San Clemente, in which the enclosed quire also remains. (3) The transept, even in Rome, was an exceptional arrangement, and does not appear in the basilicas of Ravenna.