We have elsewhere (Graf & Cummings, 2006b) demonstrated the evidence supporting the higher classification of the Unionoida in a phylogenetic context, and that article provides a detailed review of data and methods applied to date to recover the higher-level pattern of cladogenesis of freshwater mussels. Figure 2 depicts a cladogram of the unionoid families based upon our analysis; nodes for which robust phylogenetic tests are wanting are left unresolved. Areas of disagreement based