Graham Charles (2005, 20: 17-18), anticipating the subsequent use in the New Cactus Lexicon (Hunt et al. 2006), published Gymnocalycium castellanosii ssp. ferocior (H. Till & Amerhauser) G. Charles, from which we quote the text:

14264 Gymnocalycium castellanosii ssp. ferocior (H. Till & Amerhauser) G. Charles comb. nov. B: 13959 Gymnocalycium mostii ssp. ferocior H. Till & Amerhauser, Gymnocalycium 15(1): 435 (2002). T: AR, Córdoba, Agua de Ramon, H. Borth s.n. (CORD). This transfer was suggested by Massimo Meregalli (pers. comm.). The taxon has the distinctive glossy seeds of G. castellanosii and occurs west of the G. mostii populations, on the border of Córdoba and La Rioja, nearer to the distribution of G. castellanosii ssp. castellanosii. Cultivated material certainly resembles G. castellanosii more than G. mostii. 

Our surveys in the Cañada Larga, locality between Tuclame and Agua de Ramón, area of the discovery of Gymnocalycium mostii ssp. ferocior H. Till & Amerhauser, now Gymnocalycium castellanosii ssp. ferocius (H. Till & Amerhauser) G. Charles, and subsequent surveys of the taxon, showed the presence of populations of Gymnocalycium castellanosii Backeberg (A&M 405, photos 01-20), in all of the area involved. Even the form ferocius (photo 19-20) is identifiable among these, which appears to us to be only a population with a stronger and more regular spination, compared to other forms of the variable G. castellanosii. It is not surprising that the seeds of the form ferocius (photo 13) are similar to those of G. castellanosii (A&M 419, photo 103), whereas we are faced with the same taxon, and that the two forms coexist in the same area; unlike Gymnocalycium mostii (Gürke) Britton & Rose, which is not present in the area. From the findings noted it appears that G. castellanosii ssp. ferocius has no relationship with G. mostii, but appears to be only a synonym of G. castellanosii. (June 2013)