Forget the Pyramids: That stately, sand-dusted figure on the National Geographic cover is a babe - and she's wearing nothing but three strategically placed shells. Margaret Mead never looked like this. Then again, neither did the Geographic, whose first swimsuit issue hits the stands Saturday, 18 days ahead of Sports Illustrated's annual bikini blowout. Showing all that skin may be a stretch for a magazine whose usual cover lines run to "Indus: Clues to an Ancient Civilization" and "The Magic of Snowy Owls." Nevertheless, the editors of the venerable Geographic believe they've remained true to their mission.