1. Barbour, R.W. (1971) Amphibians and reptiles of Kentucky, University Press of Kentucky, Lexington.
2. Beer, T. (1894) Die Accommodation des Fishauges, Pfluegers Archiv. Gesanite. Physiol. Menschen Tiere, 58: 523–650.
3. Beer, T. (1898) Die Accommodation des Auges bei den Reptiles, Pfluegers Archive. Gesanite. Physiol. Menschen Tiere, 69: 507–568.
4. Bennett, A.G. and Francis, J.L. (1962) The eye as an optical system, in The Eye, vol. 4, Visual Optics and the Optical Space Sense, Davson, H. (ed.), Academic Press, New York, pp. 101–132.
5. Brown, P.K., Gibbons, I.R., and Wald, G. (1963) The visual cells and visual pigment of the mudpuppy, Necturus, J. Cell Biol., 19: 79–106.