1. Van Valen, L., Amer. Mus. Novit., 2285, 1 (1967).
2. McKenna, M., Curator, 3, 221 (1962).
3. Clemens, W., Univ. Calif. Publ. Geol. Sci., 48, 1 (1963); 62, 1 (1966).
4. Sloan, R., and Van Valen, L., Science, 148, 220 (1965).
5. Lillegraven, J., thesis, Univ. Kansas (1968).