Acanthocalycium glaucum
(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).
It is a flat globular to shortly elongated solitary cactus.
Flower: Yellow or red and yellow flowers in summer, up to 6 cm long and in diameter.
Remarks: The plant described as Acanthocalycium aurantiacum, has red flowers with orange centers.
Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz