[Parfitt B. D., Zimmerman A. D., Epithelantha, Flora of North America North of Mexico, Oxford University Press, 2003, tom 4, s. 239–240]
A description of North American Epithelantha, with a key and accounts of stems, spines, flowers, fruits, habitats and flowering times. The treatment distinguishes E. bokei and E. micromeris, and gives differences in spine covering density and flower biology.