Rabanus, a follower of Alcuin, born in 776, was the author of an interesting encyclopaedia, rejoicing in the comprehensive title, "On the Universe." This work is in twenty-two books, which are supposed to cover all possible subjects upon which a reader might be curious.... The seventeenth book is on "the dust and soil of the earth," under which uninviting head he includes all kinds of stones, common and precious; salt, flint, sand, lime, jet, asbestos, and the Persian moonstone, of whose brightness he claims 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]