Butterworth & Wallace 2004, p. 1086. Butterworth, Charles A.; Wallace, Robert S. (2004), "Phylogenetic studies of Mammillaria (Cactaceae)—insights from chloroplast sequence variation and hypothesis testing using the parametric bootstrap", American Journal of Botany, 91 (7): 1086–1098, doi:10.3732/ajb.91.7.1086, PMID21653464
Butterworth & Wallace 2004, p. 1086. Butterworth, Charles A.; Wallace, Robert S. (2004), "Phylogenetic studies of Mammillaria (Cactaceae)—insights from chloroplast sequence variation and hypothesis testing using the parametric bootstrap", American Journal of Botany, 91 (7): 1086–1098, doi:10.3732/ajb.91.7.1086, PMID21653464