ALAS, MAY AND SEPTEMBER MAY NEVER twine. America's favorite interseasonal couple, comedy superstar Jerry Seinfeld, 40, of NBC's Seinfeld, and college sophomore Shoshanna Lonstein, 19, of UCLA, have called it quits—more or less—after a closely watched 18-month romance. "That's not to say they won't see each other again," says a source close to Lonstein. "It's just that they thought they should take some time apart from each other."

Too bad, because despite the age gap, their courtship had a certain storybook charm. After meeting one glorious May day last year in New York City's Central Park, the two began dating, and the smitten Seinfeld called her "the most wonderful girl in the world."

Lonstein left George Washington University in Washington after her freshman year last summer, transferring to UCLA so she could be near Seinfeld. She found an apartment in L.A., then became a frequent visitor at his Hollywood Hills home and on the Seinfeld set. Inseparable, the couple apparently enjoyed what a friend of Seinfeld's described as "kind of an adorable, normal relationship."

Normal? Maybe not, given Seinfeld's seven-day-a-week fixation with his show, his stand-up act and his collection of German cars. Lonstein gamely put up with all of that, but she drew the line at sharing her beau's after-hours small talk with his show business pals. "They'd all go out to dinner and talk about the show or their careers," says a Shoshanna partisan. "She didn't have any interest in those things."

And, says this source, "she has a temper"—which exploded last month after Seinfeld, according to one tabloid account, proposed marriage. That was fine—but not his follow-up proposal that his fiancée sign a prenuptial agreement. Friends of the couple find that scenario dubious. As one points out, Lonstein, whose father lives in New York City and is president of a successful computer-services company, "didn't need Jerry's money."

What she may need is a respite from Hollywood. Though Seinfeld offered only a terse "no comment" through his publicist, and Lonstein isn't talking at all, one of her old classmates from George Washington says, "I think the breakup is beneficial. Now Shoshanna will have a chance to enjoy college like a normal student."