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| Abstract |
Chamid bivalves attach themselves to the substratum by the left valve in the normal form or by the right valve in the "inverse" form. However, both forms can occur in some species, including Pseudocha...ma granti from California. It has long been known that almost all chamids have only an aragonitic shell, but Chama arcana and Pseudochama exogyra have been shown to have both aragonitic and calcitic shell layers. Three additional species with the calcitic layer were recognized by the X-ray diffraction. They are Chama sp. from Japan, C. pellucida from Peru, and P. granti. For a discussion of the validity of Pseudochama Odhner, 1917, it is necessary to determine whether the calcitic layer in both normal and "inverse" forms is produced by homoplasy or synapomorphy. Although "inverse" forms of Chamidae usually show transposition of both the shells and hinge, the "inverse" form of C. pulchella from Australia exhibits transposed shells with the normal dentition. Therefore, transposition of hinge and shell can clearly occur independently of one another. In contrast to the sinistral coiling of gastropods associated with anatomical asymmetry, transposition of the Chamidae is partly associated with the shell secreted by the mantle epitherium, the hinge plate by the mantle isthmus, and anatomical organization. The partial transposition of the heterodont bivalves may be caused by the symmetrical organization and may not produce a reproductive isolation. In coiled gastropods the organization is highly asymmetrical. It may be suggested that an asymmetrical animal with transposed shell and normal anatomical organization or one with a normal shell with transposed anatomical organization may indeed have limited viability. The transposition in the coiled gastropods may distrub a free interbreeding of the normal and "inverse" forms in the species and be one example of the "hopeful monster", one of explanations of rapid speciation of punctuated-equilibrium theory.show more
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