Wow, Ross! Do you have a permit for that loaded question?
 
Considering the range of shapes, from virtually lacking any
development of a callus to something as wildly shaped as those
fossil <i>o'gormani</i>, the answer likewise (most probably) ranges
from, "Shape is not a factor" to "Shape is extremely important."
 
From the first time I started doing population counts, it was obvious
that cowries have one stable physical position, i.e., aperture down.
Pour a bunch of uncallused cowries onto a tray and note how many are
aperture down, shake the tray horizontally a couple of times and see
if just about every one isn't now in an aperture-down state.
 
I've heard some cute explanations about external cowry features
(some have already been discussed in this forum) but, in many cases,
I suspect these features simply have no real bearing on the animal's
life.
 
Heavy lateral calluses? Most probably help to keep the animal's shell
aperture down, in surging water, long enough for the animal to get its
foot out and anchored.
 
No calluses at all? The animal probably lives deeply enough in the
substrate that surge is not a factor.
 
Haven't seen the monster fossil cowry recursors? (Images available at
http://www.geocities.com/~makuabob/NSN153CY.HTM#I )
What purpose could those odd ridges and long knobs have served?
My guess is that these things lived in a shallow area which was
often (if not constantly) choppy and wavy. The shape helped keep them
aperture down most of the time and, if a big wave did flip them over,
the rib/ridge made it more likely that they would quickly roll over
to the slot-down position again. Note (in the above images) how
unstable these things look on their dorsum and how 'settled' they appear
when aperture down.
 
There is much more to say about shapes (and coloration) but I'm due
elsewhere at present.
 
Aloha,
 
makuabob (a.k.a. Bob Dayle)