keenly felt than when it appears in foliate forms or conventional
patterns. Diptychs being often taken in considerable numbers and
set into large works of ivory, has led some authorities to suppose
that the Ravenna throne was made of such a collection; but this
is contradicted by Passeri in 1759, who alludes to the panels in
the following terms: "They might readily be taken by the ignorant
for diptychs.... This they are not, for they cannot be taken from
the consular diptychs which had their own ornamentation, referring
to the consultate and the insignia, differing from the sculpture
destined for other purposes. Hence they are obviously mistaken who
count certain tablets as diptychs which have no ascription to any
consul, but represent the Muses, Bacchantes, or Gods. These seem
to me to have been book covers." Probably the selected form of an
upright tablet for the majority of ivory carvings is based on
economic principles: the best use of the most surface from any
square block of material is to cut it in thin slices. In their
architecture the southern mediæval builders so treated stone, building
a substructure of brick and laying a slab or veneer of the more
costly material on its surface: with ivory this same principle
was followed, and the shape of the tusk, being long and narrow,
naturally determined the form of the resulting tablets.

The Throne of Ivan III. in Moscow and that of St. Peter in Rome
are also magnificent monuments of this art. Ivory caskets were the
chief manifestation of taste in that medium, during the period of
transition from the eighth century until the revival of Byzantine
skill in the tenth century. This form of sculpture was at its best
at a time when stone sculpture was on the decline.

There is a fascinating book cover in Ravenna which is a good example