HOLMAN, J. A. 1964. Fossil snakes from the Valentine formation of Nebraska. Copeia 1964:631-637. MAO, S., AND H. C. DESSAUER. 1971. Selectively neutral mutations, transferrins and the evolution of natricine snakes. Comp. Biochem. Physiol. 40A: 669-680. MINTON, S. A., AND S. K. SALANITRO. 1972. Serological relationships among some colubrid snakes. Copeia 1972:246-252. ORTENBURGER, A. I. 1928. The whip snakes and racers genera Masticophis and Coluber. Mem. Univ. Mich. Mus. 1. PEARSON, D. D. 1966. Serological and immunoelectrophoretic comparisons among species of snakes. Bull. Sero. Mus. 36:8. ROSSMAN, D. A., W. G. EBERLE, S. MAO, H. C. DEsSAUER AND E. V. MALNATE. 1972. Relationships . . ssil s a es fro the Valenas a. opeia 1964:631-637. . . . 1971. Selectively , s erri s and the evolution . . ioc e . Physiol. 40A: of water snakes (genus Natrix) a progress report. Herp. Rev., 4:127. SAVAGE, J. M. 1966. The origins and history of the Central American herpetofauna. Copeia 1966: 719-766. TIHEN, J. A. 1964. Tertiary changes in the herpetofaunas of temperate North America. Senck. Biol., 45:265-279. UNDERWOOD, G. 1967. A Contribution to the Classification of Snakes. British Mus. Nat. Hist. London.
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Manooch et al. (1976) studied this question.