1 shell retractor muscles are described on an internal mould of Porcellia woodwardi, a pleurotomariacean gastropod from the Carboniferous of England. The scars are located at the junction between the umbilical wall, and the apical surface and the basal surface, respectively. Similar positioning of muscle scars in Bellerophon of the same age reflects morphological convergence of the planispiral, anisostrophic Por-cellia with the planispiral, but isostrophic Bellerophon. It is concluded that the shape of muscle scars, in detail, can not contribute to solving the question of the systematic position of Bellerophon.
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John S. Peel (1986) studied this question.
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