S. John in Trullo belongs to the ordinary 'four column' type of church building, and has a narthex. Its three apses are semicircular both within and without, presenting the only instance in Constantinople of apses semicircular on the exterior. The central apse projects m. 3 beyond the body of the building, and was lighted by a large but low window, divided into three lights by two pilasters crowned with carved capitals (for details see Fig. 68); the diaconicon has been built up to form the mihrab of the mosque; the prothesis, to the north, has a barrel vault. [Illustration: FIG. 69.] The drum dome is octagonal, with eight ribs and as many windows. It seems large for the size of the church, and is lower than usual inside. The windows do not cut into the exterior cornice of the dome. Originally the dome arches rested on four piers or columns, but these have been removed in the course of Turkish repairs, and the dome arches are now supported by beams running across the church, under the impost of the arches. The arms of the cross to the north and south have barrel vaults, and the walls are pierced by triple windows. Two capitals built into the exterior face of the northern wall, and marked with a cross, were doubtless the capitals of the shafts which divided the northern window into three lights. The western arm of the cross is covered by the roof of the narthex, and lighted by a small round-headed window above it. The small narthex is in three bays, covered with cross-groined vaults. It is not probable that the church was converted into a mosque before 1591, when the patriarchal seat was removed from the Pammakaristos to S. Demetrius beside the Xyloporta. Nor could the conversion have been later than 1598, the year in which Achmed Pasha--who converted the building into a mosque--died.[333] [Illustration: FIGS. 70 AND 71.] [322] Phrantzes, p. 307. [323] Patr. Constantius, p. 80. [324] _Tagebuch_, p. 456. On the way eastwards from the residence of the Moldavian agent (Bogdan Serai), says Gerlach, 'Auf diesem Spazier-weg hat mir Theodosius auch den Trullum auf der Seiten des Patriarchats gegen dem Sultan Selim gewiesen. Welches vor diesen ein sehr weiter Platz gewesen, nun aber nichts mehr da als ein rundes getäffeltes Haus, wie ein kleines Kirchlein ist.' Cf. his statement reported by Crusius in _Turcograecia_, p. 189: 'Patriarchatui contiguum est monasteriolum Joannis Baptistae a Graecis sanctimonalibus inhabitatum.' [325] Phrantzes, p. 307; cf. _Turcograecia_, p. 189. [326] It was also styled [Greek: Ôaton], 'the Oval,' after the form of its roof or of the body of the building itself (Synax., Sept. 14). _Vita Stephani._ For the [Greek: Ôaton], see Labarte, _Le Palais impérial de Cons'ple_, pp. 62, 121, 122, 186. [327] _Vita Stephani Junioris_, Migne, _P.G._ tom. 100, col. 1144 [Greek: en tô hierô palatiô, entha epilegetai ho Troullos hoper hêmeis Ôaton kaloumen].