An Extraordinary Egg

``On Pebble Island, there lived three frogs: Marilyn, August, and one who was always somewhere else.'' Like the amiable animals in Lionni's previous, inimitable fables, these anthropomorphized amphibians have a quiet but memorable adventure. In the spotlight is the roving frog, Jessica, who is ``full of wonder,'' and proclaims everything she finds--even common pebbles--``extraordinary.'' When she comes across a stone that is ``perfect, white like the snow and round like the full moon on a midsummer night,'' she lugs it home, whereupon Marilyn, ``who knew everything about everything,'' announces that it is a chicken egg. ``I was right! It is a chicken!'' she exclaims smugly when the egg hatches and a baby alligator emerges. Kids will giggle at the frogs' repeated references to the friendly newborn as ``the chicken.'' They'll be even more tickled when the frogs chuckle at the ``mother chicken'' who, finally reunited with her offspring, greets her ``sweet little alligator.'' ``What a silly thing to say!'' concludes the omniscient Marilyn. In his 40th book, Lionni is in typically fine form. Ages 3-7. (Apr.)