To quickly take the measure of Prelutsky's new collection of humorous verse, his claim to parity with America's best in the class, you might skim the index of first lines, concentrating on the letter I--from ""I am a paramecium, I am Ebenezer Bleezer, I am falling off a mountain, I am flying! I am flying"" to ""I've got an itch, a wretched itch"". . . via ""I'd never dine on dinosaurs, I'd never eat a beet because, I'm a basic boneless chicken."" Prelutsky's a natural rhymester. He has a keen sense of what tickles kids. His rhymes are infectious, his verses ineradicable: ""Nine mice on tiny tricycles/went riding on the ice,/they rode in spite of warning signs,/they rode despite advice."" He slips into nonsense without your hardly knowing it: ""Nine mindless mice, who paid the price,/are thawing slowly by the ice,/still sitting on their tricycles/ . . . nine white and shiny micycles."" Like others of his kind, he makes bold with words: ""I praise the hippopotamus,/ I celebrate the bat,/ I hold the bream in high esteem--/I DO NOT LIKE THE RAT!"" (""I cotton to the octopus,/I tolerate the gnat. . ."") On this occasion and others, the typography is part-and-parcel of the entertainment. But the pair-up with Stevenson is the coup here: both have an offhand drollery that knows no age distinctions. A fat, squat book, full of jolly rhymes and juicy drawings, that's fun just to have around.