The xylophone plays a fast tune which sounds like skeletons playing. Saint-Saëns is making a joke about himself, because he uses a tune from one of his own works: the "Danse Macabre". There are bits of other tunes as well: "Ah! vous dirai-je, Maman" (known in the English-speaking world as "Twinkle Twinkle Little Star"), the French nursery rhymes "Au Clair de la Lune" and "J'ai du bon tabac", the popular anthem "Partant pour la Syrie" as well as the aria "Una Voce Poco Fa" from Rossini's "Barber of Seville". Saint-Saëns was saying that these tunes were old-fashioned (like fossils).