Dr. Ellis L. Yochelson writes: I proposed a list of features considered common to all or most molluscs (Yochelson 1961) in an attempt to define that phylum without reference to soft parts; there has been no discussion of this approach, nor of the features listed. As a prime consideration I would make a distinction between phosphatic and non-phosphatic hard parts: phosphatic hard parts do not occur in molluscs. The late Cambrian molluscan class Mattheva has no relationship to the various asymmetrical phosphatic sclerites discussed by Matthews & Missarzhevsky (1975, p. 298–9); the latter separate these on morphological rather than chemical grounds. It does not follow, however, that all fossils with hard parts of calcium carbonate are molluscs, and there is no consensus among workers as to what fossils should be included in the phylum or excluded from it. Thus, Runnegar et al. 1975 suggest that Hyolithes and its allies constitute an extinct phylum, whereas Marek & Yochelson (1976) continue to place them as an extinct class of Mollusca. At least two diverse views of the evolution of the Mollusca have been presented by palaeontologists. Runnegar & Pojeta (1974) considered all extant classes of molluscs, except Scaphopoda, began at various times during the Cambrian and recognized no extinct classes except the Rostroconchia. Yochelson (1963) suggested a two-step evolution with a number of extinct classes early in the Palaeozoic and extant classes of molluscs first appearing in or after the late Cambrian.

The Gastropoda is an extant class defined by peculiar

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