'bead and reel.' The arches have radiating voussoirs, and the arch spandrils and the frieze under the cornice are inlaid with scroll and geometrical designs in black, white, and coloured marbles. The cornice is of grey marble with a 'cyma recta' section, and is carved with an upright leaf.[537] On the eastern walls of the north and south cross arms, and flanking the apse, eikon frames similar to those in the Diaconissa (p. 186) are inserted. The northern frame encloses a mosaic figure of Christ holding in His hands an open book, on which are the words, 'Come unto Me all ye who labour and are heavy laden.'[538] In the corresponding frame to the south is the figure of the Virgin, and, above it, an arch of overhanging acanthus leaves enclosed within a square frame with half figures of angels in the spandrils. The arch encloses a medallion bust, the head of which is defaced, but which represented the Saviour, as is proved by the indication of a cross on the aureola. The spaces at the sides of the medallion are filled in with a pierced scroll showing a dark slab of porphyry behind it, making a very beautiful arrangement. These frames are distant from the eikonostasis, which stretched across the front of the bema arch, nearer to the apse. On the south side are two doors leading to the parecclesion, and on the north side above the cornice is a small window from the north gallery. The dome rests on a ribbed drum of sixteen concave segments, and is pierced by eight windows corresponding to the octagonal form of the exterior. The original crown has fallen and been replaced by the present plain Turkish dome. The prothesis and the diaconicon are represented by chapels to north and south of the apse. As already stated, they do not now communicate with the bema, although the position of the old passages between them and the bema is marked by niches in the marble revetment. From the fact that the Byzantine marble work is continued across these passages it is evident that the chapels were cut off from the apse in Byzantine days. The north chapel is covered by a drum dome of eight concave sections, and is entered from the lower story of the gallery on the north side of the church. It should be noticed that the chapel is not placed axially to this gallery. The south chapel is covered by a plain drum dome, and is now entered from the parecclesion, evidently as the result of the alterations made when the parecclesion was added. The exterior is very simply treated. The side apses show three sides of an octagon. The central apse has five sides of a very flat polygon, and is decorated with hollow niches on each side of a large triple window. It was at one time supported by a large double flying buttress, but the lower arch has fallen in. As the buttress does not bond with the wall it was evidently a later addition.