  Let O be a maximal order in a totally indefinite quaternion algebra over a
totally real number field. In this note we study the locus Q_O of quaternionic
multiplication by O in the moduli space A_g of principally polarized abelian
varieties of even dimension g with particular emphasis in the two-dimensional
case. We describe Q_O as a union of Atkin-Lehner quotients of Shimura varieties
and we compute the number of irreducible components of Q_O in terms of class
numbers of CM-fields.
