>  A better question may be "If you have a group of genera which you deem
>  sufficiently similar to warrent grouping them together in a less than
>  family - oriented manner, how do you decide whether to call the
>  resulting taxa a "tribe" (a term very seldom used in molluscan taxonomy,
>  for some reason), or a "sub-family"??  Is there any hard and fast
>  difference between the two taxonomic categories, other than that the
>  "tribe " is deemed to be a slightly more cohesive or tighter grouping
>  than that of "sub-family"?

My impression and usage is to use a subfamily first and then a tribe.  I.e., if a family has well-marked divisions, they would be subfamilies.  If the family has well-marked divisions that have well-marked divisions, then there would be subfamiles and tribes.

Another component is historical.  If a group has had a lot of names applied to it that have received a lot of use, it may be most practical to continue to recognize the names at some taxonomic level.  E.g., in the unionoids, different workers have recognized the Lampsilidae/inae/ini.  People who work with them have a general idea of what this group is, whatever rank one assigns it.  In turn, this group is part of a larger Amblemoidea/idae/inae, etc.

    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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