Appendix S1. Material examined (institutional abbreviations: FMNH, Field Museum of Natural History; HDW NIU, Harlan D. Walley Northern Illinois University; INHS, Illinois Natural History Survey; KUMNH, University of Kansas Museum of Natural History; UIMNH, University of Illinois Museum of Natural History; UMMZ, University of Michigan Museum of Zoology; UTA, Amphibian and Reptile Diversity Research Center, University of Texas at Arlington).

Appendix S2. Sources of information on hemipenis morphology of North American natricine snakes (Fig. 3).

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