The Extinct Alphabet Book

``H'' is for Hallucigenia--a creature so startling in appearance that the scientists who discovered its fossils thought they were hallucinating. ``X'' is for Xerces Blue--a gossamer-winged butterfly that dwelled on one particular hill in San Francisco, until urban expansion destroyed its habitat. This alphabetized assembly of bygone species mixes the ancient and contemporary, the bizarre and beautiful. Pallotta's conversational text swings between scientific (``Fossilization occurs when minerals replace what used to be the bones of an animal'') and silly (``K is for Kaka . . . If we could go back in time and talk to this bird it would probably say, `I wish I had a different name' ''); between an activist's gravity and a punster's levity. Masiello's exotically colored oil paintings lend the text needed weight and, if readers look closely, wit. Although the book's tone and information are well beyond the comprehension skills of letter-learners, the alphabet device serves to marshal the roster of extinct species without rebuffing adventurous preschoolers. Nothing so obvious as a dinosaur is included, but neither are pronunciations of words sure to fluster even the most confident readers. Ages 3-8. (Aug.)