.
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).]

From its distinguishing feature this type has been styled the 'four
column' plan. It appears in many Constantinopolitan churches, as, for
example, S. Theodore (p. 248) and S. Saviour Pantepoptes (p. 214). The
cross arms are not always equal, and may be covered with barrel vaults
(p. 214) or with cross-groined vaults (p. 198). The bema is usually a
bay added to the eastern arm. The angle chambers have either
cross-groined vaults or flat dome vaults. In general the churches of
th