Found in Uruguay and Brazil, Parodia mammulosa is a solitary globular cactus to 3 inches across. It is closely related to Parodia turecekiana, which has longer more papery spines. It produces the standard yellow flower in spring. Neither of the two I have in my garden (above) have yet flowered for me. It could be they're too small yet.
     Update May 2007. The larger one has flowered.