BYZANTINE ARCHITECTURE 17 It is most unfortunate that the domes of these three domed cross churches have been altered, especially as the domes of S. Mary Diaconissa and S. Theodosia are larger than any of the later domes except the large oval dome on the central church of the Pantokrator which is almost of the same size. It is therefore now difficult to say what was the precise form of the original domes. Most probably they were polygonal drum-domes, and their collapse owing to their size may well have led to the small drum-domes of later times. Though not strictly Byzantine these Turkish domes are of interest as showing the development of Byzantine forms under Turkish rule, and that reversion to the earlier drumless dome which is so marked a feature of the imperial mosques of the city. Domes are either eight, twelve, or sixteen sided, and usually have a window in each side. These numbers arise naturally from setting a window at each of the cardinal points and then placing one,' two, or three windows between, according to the size of the dome. Internally the compartments are separated by broad, flat ribs, or are concave and form a series of ridges on the dome which die out towards the crown. In sixteen-sided domes of the latter type the alternate sides sometimes correspond to the piers outside, so that the dome which has sixteen sides within shows only eight sides without, as in the narthex of S.Theodore (p. 246). The octagonal dome of the Myrelaion FIG. jo.—THE DRUM DOME.