BioStor
 | A critical review of the characters and variations of the snakes of North America E D Cope (1892) Proc. US Natl. Mus. 14: 589-694 |
 | Variations and relationships in the snakes of the genus Pituophis Olive Griffith Stull (1940) Bulletin of the United States National Museum 175: 1-225 |