Ihlenfeldtia vanzylii
(This plant comes in a 3.5" pot. You will receive a very similar plant to the one in the picture. It may or may not be blooming at the time of your purchase. This plant is shipped in its pot to prevent any damage to the roots).
Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz
