1. Becher, A. (1923).Arch. Naturgesch. (9). Quoted by Huxley.
2. Chubb, S. H. (1927). See Matthew and Chubb.
3. Huxley, J. S. (1932).Problems of Relative Growth. With 105 illustrations. Pp. 276. New York: Dial Press.
4. Matthew, W. D. andChubb, S. H. (1927).Evolution of the Horse. Amer. Museum Nat. Hist. Guide Leaflet series, No. 36. Fifth ed. revised. Pp. 62. New York City.
5. Osborn, H. F. (1929). “Titanotheres of Ancient Wyoming, Dakota and Nebraska.”U.S. Dept. Int. Geol. Survey, Monograph No. 55, 2 vols.