ABSTRACT
As with all historical processes, the exact course of evolution could never be predicted. As Stephen Jay GoulD expressed it, the result would not be the same if the tape could be run twice. Nevertheless there are general rules set by the limitations of a given bauplan, as well as by the functional continuity required for evolutionary transformations in a structured biosphere. Because of their basic uniformity of morphology and because most adaptational types survive to the present day, bivalves are particularly suited for designing a road map (Pl. 15.1) that has been independently followed in the evolutionary history of different clades. It should be understood, however, that such a scheme cannot replace actual cladistic analyses. Rather than ancestry, it emphasizes the adaptational attractors and the new evolutionary opportunities at every station, some of which may not have been explored by a particular group due to specific constraints or by mere chance.
