  We investigate the clustering structure of species sampling sequences
$(\xi_n)_n$, with general base measure. Such sequences are exchangeable with a
species sampling random probability as directing measure. The clustering
properties of these sequences are interesting for Bayesian nonparametrics
applications, where mixed base measures are used, for example, to accommodate
sharp hypotheses in regression problems and provide sparsity. In this paper, we
prove a stochastic representation for $(\xi_n)_n$ in terms of a latent
exchangeable random partition. We provide explicit expression of the EPPF of
the partition generated by $(\xi_n)_n$ in terms of the EPPF of the latent
partition. We investigate the asymptotic behaviour of the total number of
blocks and of the number of blocks with fixed cardinality in the partition
generated by $(\xi_n)_n$.
