Angwantibo. . . Boobook. . . Coypu. . . Unfaltering, in stately rhyme, Farber parades one uncommon beast after another across Boston Common, each animal towing the next in an alphabetical sequence that holds up all the way through Xenopus, Yaguarundi and Zibet--until, at dusk, all form ""sweet and slow, a circular tow,/ round as the moon that leaned to blow/ its beams upon Boston Common."" A rarity indeed, but Farber's unostentatious way with words and sure sense of mood make it more than a clever novelty. And the green-toned illustrations, with his characteristic combination of softness and precision which Lobel informs here with an ""antiquarian ambience,"" make of that sweet slow circling a magical dream you won't want to end.