Mosco A., Zanovello C., A phenetic analysis of the genus Thelocactus, Bradleya, 2000, 18:29-54, doi: 10.25223/brad.n18.2000.a6
A comparison of 65 morphological traits, taxon clustering and a discussion of variation and species boundaries in Thelocactus. The paper describes the treatment of the T. bicolor group and includes a new combination for subspecies heterochromus.