Found across a wide range of South America (Bolivia, Argentina, Uruguay, and Paraguay). Echinopsis oxygona is a clumping globular cacti to 12 inches tall. It can form clumps 2 feet across. The type species has flowers that vary from magenta, red, orange or yellow. The pure white one pictured here is sometimes considered a seperate species know as Echinopsis multiplex. But it's listed as synonymous with E. oxygona by Desert Tropicals.
      I got a bunch of these, this 5 inch wide head and a dozen offsets, from a trade with "Xenomorf", a fellow member of the Davesgarden.com gardening forums. This big one will probably go in the garden somewhere. He's also allowed me to use his pictures of the magnificent flower. Thanks Xeno :-)