The classification of the Foraminifera, a widely distributed group of largely marine protists, has traditionally been based on morphological characters. The most important of these are the composition and structure of the shell or "test." Here, we use both phylogenetic analysis of the genes for small subunit rRNA and beta-tubulin and ultrastructural analysis to document a reversion in wall type from more derived calcareous tests to an agglutinated test. These data indicate that the genus Miliammina, and possibly other members of the Rzehakinidae, should be placed in the Order Miliolida as opposed to their current assignment in Order Textulariida. We also address the effects this reversion may have had on the ability of rzehakinacids to effectively colonize marginal marine environments. Finally, the hypothesis that some multilocular agglutinated foraminiferans descended from calcareous lineages has implications for interpretation of the foraminiferal fossil record.