  Here we focus on the geometry of $\pdgbar$, the compactification of the
universal Picard variety constructed by L. Caporaso. In particular, we show
that the moduli space of spin curves constructed by M. Cornalba naturally
injects into $\pdgbar$ and we give generators and relations of the rational
divisor class group of $\pdgbar$, extending previous work by A. Kouvidakis.
