The newspaper ads had been very specific, almost stern. "Please save our time and yours," they had admonished. "Do not come to these auditions unless you are an excellent skater, capable of traveling at high speeds, turning, spinning and jumping."

Any number of the women in tights busily stretching their hamstrings on the floor of the Roxy rink on West 18th Street this morning ignored that written finger-wagging. This was "Starlight Express," the new musical from Andrew Lloyd Webber, the man who brought you "Cats," now and forever. "Starlight Express," still selling out in London after 14 months, with rehearsals for a U.S. tour planned for January. "Starlight Express," emphasis on the star. If everyone in the cast of 56 has to dance and sing on wheels, and impersonate a locomotive, well, that seemed a piddling point.

Elizabeth Preim, a cabaret singer who hasn't been on roller skates since she was a counselor at Camp Pinecrest in Fort Lauderdale, Fla., supplemented her leotard with thick blue kneepads bought especially for the occasion. "I don't want to get hurt," she confided, assuring the others that "it's the guys who do the really tricky skating."

"This man wrote 'Cats,' " said Jacqueline Malick Lowell, 22, a singer who recently portrayed a snowman in an insulation commercial but doesn't skate. "I just want him to hear me sing."

Actually, Lloyd Webber was not on hand for the audition but director Trevor Nunn, choreographer Arlene Phillips, the skating and music directors and the stage manager were finishing styrofoam cups of coffee and taking their places at a rinkside table. Assistant stage manager Mark Torres held numbered cards filled out by 96 hopefuls (No. 1 was Amelia Prentice, an ex-Ice Capades skater who heretofore had never laced up a pair of roller skates but figured she could wing it). One hundred fifty male skaters had shown up Monday, another 150 in Los Angeles last weekend. If the coasts don't yield enough singing, dancing skaters, the search will extend into the heartland (Chicago and Houston are possibilities).

"Okay, ladies, freestyle warm-up," called Torres. The first 50 auditioners streamed out onto the Roxy's polished floor, edged with fuchsia and purple neon rods, to show off their spins and splits. To catch a caster's eye, any stunt was fair game, from wearing a neckful of rhinestone chokers to pirouetting mere inches from the director's nose, so long as the smile never left one's lips.

Perhaps the most noticeable skater was Debbie Merrill, encased from the straps of her black-and-silver beaded bikini to the wheels of her skates in a black fishnet body stocking studded with rhinestones. It was made for her, said Merrill, who also wore false eyelashes and a small gold crucifix, for a show called "Spice on Ice."

Arlene Phillips, a sprite in sneakers and unruly red hair, taught the assembled group a churning dance combination. "Up in, out in, down in, out in," she chanted, pumping her arms. "Now switch the feet right, left, right, left." They went through the steps half-a-dozen times to a number from the show that featured clanging locomotive bells and lyrics on the order of, "Wooo wooo, nobody can do it like a steam train."

The skaters and singers retired to one side of the rink to practice the combination, the experienced dancers to the other side to add several more bars of gyration and a brief glide. The dancers had it, in the hips, in the shoulders, in the practiced toss of their heads. The skaters, most of them Roxy habitue's with little training, couldn't remember the steps as well but some of them could breakdance on wheels. Despite some wobbles, no one actually landed on the floor. The casting team, taking notes on the numbered cards, watched each group perform, then asked for groups of four, then had each foursome repeat the combination.

"I think positive," said Merrill, waiting her turn with her skating pal Janet Jones. "I believe in myself and in God."

"They put me in the tough group, but I think I did okay," worried Amelia Prentice, who recognized Merrill from an ice show that both performed with a few years ago. Their professional paths have since diverged: Prentice, 23, will play Juliet at a Renaissance festival in Tuxedo, N.Y., this summer and has just tested for "Search for Tomorrow." Merrill does lip-synced Cyndi Lauper impersonations on skates, complete with glad rags and pounds of jewelry, at discos and children's birthday parties. She was just back from the Concord resort in the Catskills. "They loved it," she said. "I'm trying to get on 'Puttin' on the Hits' this week."

While the winnowing continued, Steve Leber kept his ear to the phone in the Roxy office above the rink. Leber, along with the Robert Stigwood Organization and Lloyd Webber's The Really Useful Co., will produce the American version of "Starlight Express."

It remains a somewhat vague venture, its dates and sites still undetermined, but its producers see it as a breakthrough in theatrical democracy. Unlike the London production, it will travel the country, being booked into arenas ("like the Cap Centre," Leber said) instead of taking up residence in an expensive New York theater. "It's a major step, going from a Broadway concept to an arena concept," Leber offered between calls, adding that the logistics would be "staggering" and that press conferences would be forthcoming.

Downstairs in the lounge, as Torres called for the remaining auditioners to begin the process all over again, Prentice rolled up to Merrill. "I got tapped; I have to sing at 1 o'clock," she gasped. "Isn't that hysterical?"

"Great!" Merrill responded with a hug.

"I wish you had."

"My costume wasn't bright enough," Merrill deadpanned. "But they're going to put us on 'Entertainment Tonight.' "