A large number of domes thus raised high above their pendentives or corbellings are really of a class whose claims to the name of dome are somewhat ambiguous. I refer to those whose horizontal section is not circular, but polygonal (Figs. 418, 419). Strictly speaking, this is a variety of groined vault; it is generated by the intersection of several barrel-vaults, springing from the horizontal tops of the surrounding walls. Now, my definition of a dome was a vault generated by the revolution of an arch on its vertical axis. If this were an exhaustive definition, it would follow that the vaults under consideration are no domes at all; yet they look so much like them, and as the number of the sides of the polygon increases, actually approach so closely to the genuine dome, that it would be affectation to deny them the name. They may form the covering of any rectilinear figure at all approaching to regularity of form; as the triangle, the square, the canted square, polygons, either regular or elongated, oblongs or parallelograms of any kind; but the usual form is the octagon or other polygon, and for our general purpose it may be best to limit them to figures capable of being inscribed in a circle or an ellipse. I have introduced this variety of dome as occurring in those which are raised high above their pendentives or corbels. They occur, however, in numerous positions. The greatest I know of is that of the Cathedral at Florence, of which I shall have to speak more in detail in my next lecture. [Illustration: Fig. 420. Plan, SS. Sergius and Bacchus. (_From Fergusson._)] [Illustration: Fig. 421. Section, SS. Sergius and Bacchus, Constantinople. (_From Fergusson._)] But to return to the Pendentive Dome. The Pendentive Dome, though occasionally used at an earlier period, established itself as the leading feature of a style about the time of the Emperor Justinian, and its central seat was Constantinople. The earliest, or _about_ the earliest, church now remaining in that city, seems to be that of SS. Sergius and Bacchus (Figs. 420, 421). Its dome is supported by an octagon. It appears itself to be conchoidal in its horizontal section, and to be supported on sixteen small pendentives. It bears considerable resemblance to the Temple of Minerva Medica, but is really less developed than the baptistery at Ravenna, which dates full half a century earlier.