aims that it
"waxes and wanes with the moon." Later he devotes some space to
pearls, crystals, and glass. Metals follow, and marbles and _ivory_,
though why the latter should be classed among minerals we shall
never understand.

[Illustration: IVORY TABERNACLE, RAVENNA]

The Roman diptychs were often used as after-dinner gifts to
distinguished guests. They were presented on various occasions.
In the Epistles of Symmachus, the writer says: "To my Lord and
Prince I sent a diptych edged with gold. I presented other friends
also with these ivory note books."

While elephant's tusks provided ivory for the southern races, the
more northern peoples used the walrus and narwhale tusks. In Germany
this was often the case. The fabulous unicorn's horn, which is so
often alluded to in early literature, was undoubtedly from the
narwhale, although its possessor always supposed that he had secured
the more remarkable horn which was said to decorate the unicorn.

Triptychs followed diptychs in natural sequence. These, in the Middle
Ages, were usually of a devotional character, although sometimes
secular subjects occur. Letters were sometimes written on ivory
tablets, which were supposed to be again used in forwarding a reply.
St. Augustine apologizes for writing on parchment, explaining, "My
ivory tablets I sent with letters to your uncle; if you have any
of my tablets, please send them in case of similar emergencies."
Tablets fitted with wax linings were used also in schools, as children
now use slates.

Ivory diptychs were fashionable gifts and keepsakes in the later
Roman imperial days. They took the place which had been occupied
in earlier days by illuminated books, such as were produced by
Lala of Cyzicus, of whom mention will be made in connection with
book illuminators.

[Illustration: THE NATIVITY; IVORY CARVING]

After the triptychs came sets of five leaves, hinged together;
sometimes these were arranged in groups of four around a central
plaque. Often they were intended to be used as book covers.
Occasionally the five leaves were made up of classical ivories
which had been altered in such a way that they now had Christian
significance. The beautiful diptych in the Bargello, representing
Adam in the Earthly Paradise, may easily have been originally
intended for Orpheus, especially since Eve is absent! The treatment
is rather classical, and was probably adapted to its later name.
Some diptychs which were used afterwar