72 Christopher W. Brown and Carl H. Ernst 



phases — the typical dark one, as tested for in this study, and a yellow 

 one. There was no possible way, in certain cases, for this study to 

 determine whether a light-colored C. h. horridus specimen was truly the 

 yellow phase, as opposed to a badly faded normal dark phase, or the 

 color of a typical C. h. atricaudatus specimen. Use of ground color as a 

 discriminating character obviously requires fresh or living specimens 

 and an improved coding scheme. 



The evidence presented here suggests that, on the basis of differen- 

 ces in adult size and pattern, two subspecies of Crotalus horridus (as 

 described by Gloyd 1940) occur east of the Appalachians. There the 

 races are clearly more distinct than in the western populations. Stand- 

 ard morphological characters alone are not sufficient to separate the 

 two taxa; rather, adult size and pattern differences, in conjunction with 

 the number of dorsal scale rows and ventral scutes, best discriminate C. 

 h. horridus from C. h. atricaudatus. This combination of size, pattern, 

 and morphological differences needs to be examined in western popula- 

 tions, preferably on living or freshly collected specimens for accurate 

 determination of color and pattern. We feel that the results of our study 

 are preliminary and that a comprehensive study of variation throughout 

 the entire range of C. horridus is needed. 



ACKNOWLEDGMENTS.— We thank C. J. McCoy, Carnegie 

 Museum of Natural History (CM); William M. Palmer, North Carolina 

 State Museum of Natural History (NCSM); and George R. Zug and W. 

 Ronald Heyer, National Museum of Natural History, Smithsonian 

 Institution (USNM), for allowing us to examine their specimens of Cro- 

 talus horridus. We are also grateful to Madeleine Kennedy and Charles 

 Crumly for their unselfish help with the analyses and interpretation. 



LITERATURE CITED 



Anderson, Paul. 1965. The Reptiles of Missouri. Univ. Missouri Press, Columbia. 



Conant, Roger. 1975. A Field Guide to Reptiles and Amphibians of Eastern 

 and Central North America. Houghton Mifflin Co., Boston. 



Dixon, W. J., and M. B. Brown (editors). 1979. BMDP-79 Biomedical Compu- 

 ter Programs P-Series. Univ. California Press, Berkeley. 



Dowling, Herndon G. 1951. A proposed standard system of counting ventrals in 

 snakes. Br. J. Herpetol. 1:97-99. 



Gloyd, Howard K. 1940. The Rattlesnakes, Genera Sistrurus and Crotalus. Chi- 

 cago Acad. Sci., Chicago. 



Klauber, Lawrence M. 1938. A statistical study of the rattlesnakes, V. Head 

 dimensions. Occ. Pap. San Diego Soc. Nat. Hist. 4:1-53. 



. 1972. Rattlesnakes, Their Habits, Life Histories, and Influence on 



Mankind. 2nd ed. 2 vol. Univ. California Press, Berkeley. 



