1. Grandjean, F., Bull. Géol. Soc. France, Sér., 4, 10, 496 (1910).
2. Miller, A. K., and Unklesbay, A. G., J. Paleontol., 17, 1, (1943).
3. Arkell, W. J., in Treatise on Invertebrate Paleontology (edit. by Moore, R. C.) (Univ. Kansas Press and Geol. Soc. Amer., 1957).
4. Eichler, R., and Ristedt, H., Science, 153, 734 (1966).
5. Hyatt, A., in Zittel's Textbook of Palaeontology (edit. by Eastman, C. R.) (Macmillan, London, 1900).