rical dome on pendentives; and thirdly, the hemispherical dome
with a drum interposed between it and the pendentives.

[Illustration: FIG. 8.--THE SAUCER DOME OR DOME-VAULT.]

Flat external cornices on the dome are not uncommon in the later
churches of Byzantine Greece, as in S. Sophia at Monemvasia.[27] In
Constantinople only one dome with a flat cornice can be regarded as
original, that of S. John in Trullo, a church which is exceptional also
in other respects. The many other domes in the churches of
Constantinople on high drums and with flat cornices are Turkish either
in whole or in part. The high ribless domes of the Panachrantos, for
instance, circular in plan within and without, with square-headed
windows, plain stone sill, and flat cornice in moulded plaster, may be
regarded as typical Turkish drum-domes. As will appear in the sequel,
the dome over the north church of the Pantokrator and the domes of SS.
Peter and Mark, the Diaconissa, and S. Theodosia, are also Turkish.

[Illustration: FIG. 9.--THE DOME ON PENDENTIVES.]

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.

[Illustration: FIG. 10.--THE DRUM DOME.]

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 wi