William Berryman Scott, Glenn Lowell Jepsen, Albert Elmer Wood, The Mammalian Fauna of the White River Oligocene: Part V. Perissodactyla, Transactions of the American Philosophical Society, New Series, Vol. 28, No. 5 (Jun., 1941), pp. 747-964+975-980
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Scott et al. (1941) studied this question.
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