DR. ELLIS L. YOCHELSON writes: I proposed a list of featuresconsidered common to all or most molluscs (Yochelson 1961) inan attempt to define that phylum without reference to soft parts;there has been no discussion of this approach, nor of the featureslisted. As a prime consideration I would make a distinctionbetween phosphatic and non-phosphatic hard parts: phosphatichard parts do not occur in molluscs. The late Cambrian molluscanclass Mattheva has no relationship to the various asymmetricalphosphatic sclerites discussed by Matthews & Missarzhevsky (1975,p. 298–9); the latter separate these on morphologicalrather than chemical grounds. It does not follow, however, thatall fossils with hard parts of calcium carbonate are molluscs,and there is no consensus among workers as to what fossils shouldbe included in the phylum or excluded from it. Thus, Runnegar et al. 1975suggest that Hyolithes and its allies constitute an extinctphylum, whereas Marek & Yochelson (1976) continue to placethem as an extinct class of Mollusca. At least two diverse viewsof the evolution of the Mollusca have been presented by palaeontologists.Runnegar & Pojeta (1974) considered all extant classes ofmolluscs, except Scaphopoda, began at various times during theCambrian and recognized no extinct classes except the Rostroconchia.Yochelson (1963) suggested a two-step evolution with a numberof extinct classes early in the Palaeozoic and extant classesof molluscs first appearing in or after the late Cambrian.
The Gastropoda is an extant class defined by peculiar
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This 250-word extract was created in the absence of an abstract.
