Thelocactus bicolor and T. hexaedrophorus are compared to Ferocactus hamatacanthus and Hamatocactus setispinus in 19 characters. H. setispinus has a greater similarity to T. bicolor than to the other 2 species and is placed in Thelocactus as T. setispinus (comb. nov.). The following characteristics of Thelocactus are discussed: ribs and tubercles, areoles, spines and extrafloral nectaries, epidermis, outer cell layers, druses and crystals, flowers, pollen, fruits, and seeds. Ecological characteristics, such as distribution, soils, habitat characteristics, sympatry, and associated plants, are described. The relationships and general features of each species are considered. In the section on formal taxonomy Thelocactus is redefined to include T. setispinus, T. bicolor, T. leucacanthus, T. macdowellii, T. tulensis, T. hastifer, T. conothelos, T. heterochromus, T. hexaedrophorus, T. lausseri, and T. rinconensis.