[Illustration: FIG. 4.--S. NICHOLAS, MYRA (Rott).] In Constantinople there are three churches which seem to constitute a type apart, though resembling in many ways the types just considered. They are S. Andrew in Krisei, (p. 117), S. Mary Pammakaristos (p. 150), and S. Mary Panachrantos (p. 130). In these churches, as originally built, the central dome is carried on four arches which rise above a one-storied aisle or ambulatory, allowing of windows in the dome arches on three sides--the eastern dome arch being prolonged to form the bema. The dome arches have arcades communicating with the ambulatory on the north, south, and west. The vaulting is executed either with barrel or with cross-groined vaults. These churches are evidently planned from a centre, not, like the domed basilicas, from a longitudinal axis. At the same time the absence of any cross arms differentiates them from the domed cross churches. S. Andrew, which still retains its western arcade, dates from at least the sixth century, so that the type was in use during the great period of Byzantine architecture. Indeed, we should be inclined to regard S. Andrew as a square form of SS. Sergius and Bacchus, but without galleries. The type is a natural development from the octagonal domed church with its surrounding ambulatory. [Illustration: FIG. 5.--THE CHURCH OF THE KOIMESIS, NICAEA (Wulf).] The typical late Byzantine church is a development from the domed cross plan. In three examples in Constantinople, S. Theodosia (pp. 170, 172), S. Mary Diaconissa (p. 185), and SS. Peter and Mark (p. 193), we can trace the gradual disappearance of the galleries. S. Theodosia, as has already been mentioned, has galleries in all three cross arms. In S. Mary Diaconissa they are confined to the four angles between the cross arms; SS. Peter and Mark is a simple cross plan without galleries. In later times it became customary to build many small churches, with the result that the chambers at the angles of the cross, of little account even in a large church, were now too diminutive to be of any value, and the question how to provide as much room as possible for the worshippers became paramount. Accordingly the dome piers were reduced to mere columns connected with the outer walls of the building by arches; and thus was produced the typical late Byzantine plan--at the ground level a square, enclosing four columns; above, a Greek cross with a dome on the centre. [Illustration: FIG. 6.--THE CHURCH OF THE KOIMESIS, NICAEA (Rott).]