Three-and-a-half hours of Jean Crain and Janet Leigh in one double-feature is as good an excuse as any to forget that Thursday exam for one evening.

It seems that some time ago MGM made a lot of money with a movie called "Cheaper By the Dozen," and decided to produce a sequel. Because "Son of Cheaper By the Dozen" sounded foolish, and for probably no other reason at all, it was named "Belles On Their Toes." It continues the story of a prolific female engineer, her twelve miraculously self-sufficient off-spring, and a servant named Hoagy Carmichael. It is not an insufferably hilarious picture. In the maudlin machinations of twelve small children there is probably warm, lovable, nostalgic, human appeal, if you happen to like that sort of thing.

There is hardly any sex.

The sex is saved for the second feature, "Just This Once," which has Janet Leigh in a sweater, a double-feature in itself. This picture takes the fairly standard gimmicks of a lady barrister and a spendthrift young multimillionaire and parlays them, principally by clever dialogue, into an extremely funny story. It is really a better movie than the feature.

The moral: if this review instills in you an unquenchable desire to run all the way to the Orpheum with eighty-five cents clutched in your hot, sweaty little palm, get there by eight-thirty. On second thought, try eight-fifteen. There may be a cartoon.