Mediolobivia rosalviflora pygmaea
(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. The plant is shipped in its pot to prevent any damage to the roots).
This is a very small clumping species. Some of its variable forms don't really look like the same species.
Blooming season: Flowers all tend to come in one late Spring rush, rather than spread over the Summer.
Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz