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The "horns" of rhinoceroses are made of keratin, the same substance as fingernails, and grow continuously, but do not have a bone core.
* Chamaeleonidae: Many chameleons, most notably the Jackson's chameleon, possess horns on their skulls, and have a keratin covering.
* Ceratopsidae: The "horns" of the Triceratops were extensions of its skull

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