Dear Allen;
        I think it probably has to do with energy levels: shallow-water spp have to
deal with waves, rip-tides, near-shore currents, etc., so they have to have more
robust shell structures to deal with this - and these are more often more
"compact" than the elongated shapes that deeper-water species can afford to
assume, since the energy levels are on average, much less the futher one
descends into the watery deeps.

From the Great Wet and Windy north,
Ross.

ALLEN AIGEN wrote:

I have noted that some groups, like the cones, show elongation of   deepwater
species compared to the relatively short and squat shallow  water species.  Does
anyone have any explanation for this?  Does it work  within species or just
between?  Are there other groups which this  applies to?