Shear, W.A. 2011. Cave millipeds of the United States. XI. Opiona graeningi, n. sp., a troglomorphic caseyid milliped from Siskiyou County, California, with comments on the genus Opiona Chamberlin 1951 (Diplopoda, Chordeumatida, Caseyidae). Zootaxa. 3114, 1 (Nov. 2011), 50–56. DOI:https://doi.org/10.11646/zootaxa.3114.1.5.