B is for bat./He's little and brown/and when he's asleep/he hangs upside down./ He has 38 teeth/and a small pug nose./He's hunting for insects/wherever he goes."" 26 such verses, with a minimum of nature lore and not a surprising phrase or notion in the lot, comprise this latest addition to the Mousekin shelf. The water color illustrations (of newborn birds, ""polliwogs,"" ""toadstools,"" etc.) are soft and delicate -- a little too soft and delicate for our taste, but their expressive detail and sunny clarity make them hard to dismiss. For Mousekin's old friends, who will like the pictures and won't mind the innocuous rhymes.