Last year Apple delivered a fourth and final preview of Lion to developers at WWDC on June 6, saying the same day that it would ship the operating system the next month, then met that deadline by releasing the final code on July 20, 2011.

Assuming that the past is a clue to the present, Apple will ship Mountain Lion on July 25.

As with Lion, Mountain Lion will be available only from Apple's Mac App Store, and also like its predecessor, a single purchase lets a user upgrade all personally-owned eligible machines.

Mountain Lion can be installed on Intel-powered Mac notebooks and desktops currently running either OS X 10.6 (Snow Leopard) or Lion. Other system requirements include 2GB of memory and 8GB or more of free hard-disk drive or solid-state disk (SSD) storage space. Not all such Macs, however, will run Mountain Lion. (Apple has posted a on its website.

Apple will sell Mountain Lion for $19.99, a third less than last year's Lion and 31% less than 2009's first discount-priced OS X, Snow Leopard.