III. The Cross plan. Here we have a square central area covered by a dome, from which extend four vaulted arms constituting a cross. This type also assumes two distinct forms: (1) Buildings in which the ground plan is cruciform, so that the cross shows externally at the ground level. Churches of this class are usually small, and were probably sepulchral chapels rather than churches for public worship. A good example is the tomb of Galla Placidia at Ravenna. (2) In the second form of the Cross church the cross is enclosed within a square, and appears only above the roofs of the angle chambers. An example is seen in the late Roman tomb at Kusr en Nûeijîs in Eastern Palestine. In this instance the central square area is covered with a dome on continuous pendentives; the four arms have barrel-vaults, and the angles of the cross are occupied by small chambers, which bring the ground-plan to the square. The building is assigned to the second century, and shows that true though continuous pendentives were known at an early date[10] (Fig. 8). Another example is the Praetorium at Musmiyeh, in Syria,[11] which probably dates from between 160 and 169 A.D. At some later time it was altered to a church, and by a curious foreshadowing of the late Byzantine plan the walls of the internal cross have entirely disappeared from the ground-plan. The dome rests on four columns placed at the inner angles of the cross, and the vaulted cross arms rest on lintels spanning the space between the columns and the outer walls. From these three types of building are derived the various schemes on which the churches of the Byzantine Empire were planned. Of the basilican form the only example in Constantinople that retains its original plan is S. John the Baptist of the Studion (p. 56), erected _c._ 463 A.D. The church of SS. Sergius and Bacchus (p. 70) and the baptistery of S. Sophia (p. 78) represent respectively the two varieties of the octagonal plan. In the former the dome rests on piers surrounded by an ambulatory; in the latter the dome rests upon the outer walls of the buildings. Both are foundations of Justinian the Great. Of the Cross church plan showing the cross externally at the ground level no example survives in the city. But at least one church of that form was seen at Constantinople in the case of the church of the Holy Apostles. This was essentially a mausoleum, built originally by Constantine the Great and reconstructed by Justinian to contain the sarcophagi of the sovereigns and the patriarchs of New Rome.[12] The church of S. Mark at Venice was built on the plan of the Holy Apostles. It is a cruciform church with aisles, but the galleries which might have been expected above them are omitted. The central dome rests on four piers, and four smaller domes cover the arms.