Wouldn't you just know that after a whole term vacation's worth of undersea fantasy adventures, the missing half of William's magic sea shell is found on the shore, right near school. ""How extraordinarily stupid of us, Mary, not to have looked for it on our own beach,"" says William, but then there would have been no excuse for those amazing transportations into phosphorescent seascapes, a Disney-inspired fish school, an artist's restaging of Atlantis. . . even a poster-paint Hotel Whale. The scenery is often playful and occasionally awe-inspiring, but most youngsters will find William and Mary starchy company, the seashell hunt an awkward device, and the suggestion that wishing can somehow reunite William's separated parents an outright fraud. But though William and Mary are always permitted to escape from the fantasy realm when the going gets wierd, Farmer does create surreal dream experiences that have an eerie impact. There's a disheartening gap, never more apparent then here, between this author's creative, lightly philosophical imagination and the workaday fictional frameworks she builds around it. Those who managed to get inside A Castle of Bone will probably Find their way here. . . to the tricky undercurrents beyond the surface.