The distribution of shelled molluscs in the Rappahannock estuary, Virginia, is described. Distinct assemblages of molluscs are associated with particular substrates, and the diversity within each assemblage increases from the head of the estuary toward the sea. The upstream penetration of some molluscs is related to salinity. Limits on the range of other species are caused by lack of suitable substrates, predation, and competition, as well as physical and chemical environmental parameters. The morphological characteristics of some of these molluscs at the limits of their range reveal some stunting. In other species apparent stunting results from the presence of only juvenile forms.
Detailed analysis of the relative shell growth and form, the shell ultrastructure, and the magnesium and strontium content of the calcified layers of the shell has been made in population samples of Brachidontes recurvus. Shell growth interruption and possibly shell dissolution are important in the reduced salinity part of the estuary and may be important factors controlling shell chemistry. Cluster analysis of growth and shell chemistry parameters for each population defines the association of populations living in similar environments. Morphologic characters are not very useful, but strontium and magnesium concentrations can be used to predict relative position in the estuary. Ultrastructure and chemical studies are potentially more useful paleoecologic tools than morphologic studies in this apparently conservative animal.