The grins and glares, the waves and shrugs, the cross-examina- tions -- and the hats of R. Kenneth Mundy. Not since Jackie's pillbox has anyone in Washington become so quickly associated with this form of sartorial expression.

As the Marion Barry trial -- now in its seventh week -- unfolds, barely a day passes when Mundy, the mayor's defense attorney, does not turn up at the U.S. Courthouse without a summer hat of some kind. And he rarely wears the same one twice.

There have been Panamas and boaters, gondoliers and beach hats -- no sombreros or Stetsons so far. He wore a droopy-brimmed hat with a proud crown on June 21. The next day, he chose a handsome, stiff boater. On June 26 he appeared in a modest little Panama. There has been a profusion of Panamas. Sometimes the brim is wider, sometimes the band is brighter -- or has a tiny feather, as it did on July 6.

He can look like a gambler with a winning ticket, a Broadway star about to break into song, a horse trainer who's missing his cigar, an undercover cop without a wrinkled raincoat, or simply a Southern gentleman. But Mundy seldom just looks bald -- as he is, mostly -- getting off his scooter every morning. And he never comes off like the average Washington attorney. But then they always say, trial lawyers are different.