>This raises the question - what were Linnaeus's criteria for including a species in Helix? If land, marine and freshwater shells were included, was the main consideration shape, locality, colour? And when was the decision made to designate separate genera for the different groups?<

Early workers like Linnaeus seem to have relied mainly on shape, so land, freshwater, and marine taxa were all lumped if they looked similar (E.g., he put freshwater bivalves in marine genera).  The separation was gradual; most of the genera had been split into terrestrial, freshwater, and marine groups by the early 1800's, but some authors or taxa may have been slower to reassign.  A similar phenomenon can be seen in the inclusion of land snails as "seashells" in non-scientific contexts, e.g. decoration.

This parallels the general division of Linnaeus' broad genera into more restricted ones, e.g., Tridacna separate from Chama, or Pecten separate from Ostrea (not to mention the numerous genera within Pectinoidea and Ostreoidea).

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

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