Tephrocactus geometricus f. inermis:

The appeal of this South American cactus needs very little explanation. Forming an impressive mound of stacked balls with age, Tephrocactus geometricus is one of the most unusual members of the opuntia family. This is a pup of the very same plant in the second picture. 

U.S. SHIPPING ONLY
Plants ship bare root, pot/soil not included