Pausanias Description of Greece 5.12.2

[2] They may also correct their error from the following considerations. Horns drop off animals each year and grow again; the deer and the antelope undergo this experience, and so likewise does the elephant. But a tooth will never be found to grow again, at least after the animal is full-grown. So if the projections through the mouth were teeth and not horns, how could they grow up again? Again, a tooth refuses to yield to fire; but fire turns the horns of oxen and elephants from round to flat, and also into other shapes. However, the hippopotamus and the boar have tusks growing out of the lower jaw, but we do not see horns growing out of jaws.

[2] They may also correct their error from the following considerations. Horns drop off animals each year and grow again; the deer and the antelope undergo this experience, and so likewise does the elephant. But a tooth will never be found to grow again, at least after the animal is full-grown. So if the projections through the mouth were teeth and not horns, how could they grow up again? Again, a tooth refuses to yield to fire; but fire turns the horns of oxen and elephants from round to flat and also into other shapes. However, the hippopotamus and the boar have tusks growing out of the lower jaw, but we do not see horns growing out of jaws.