Frances the Mute, the latest opus by former At the Drive-In cohorts Omar Rodriguez-Lopez and Cedric Bixler-Zavala, arrived at the perfect time; its fresh-and-wild style filled the vacuum left by a dearth of new musical movements. Too bad self-appointed trend monitors are currently working overtime to pigeonhole the group as the flag-bearers of 21st-century art rock. Granted, this view isn't wholly without merit, because the players have a taste for extended solos, and they eschew standard verse-chorus-verse structures in favor of less predictable, more free-flowing compositions. But what's best about the Mars Volta is how its music refuses to conform to any standards -- even its own. The Voltans are constantly trying to come up with something different from their previous work, and if this contributes to a sort of aural schizophrenia at times, the ailment is preferable to the by-the-numbers approach taken by the vast majority of their hard rock peers. In a world of round holes, Rodriguez-Lopez and Bixler-Zavala aren't square pegs. They're shape-shifters.