P.S.  Actually the large clam and the small one probably do not fall at the
same speed, since the small specimen has a greater ratio of surface area to
mass, and is therefore affected more by air resistance.  So the greater
force of impact of the larger clam may be the result of both greater mass
AND greater speed.

Not exactly shell-related, but related to the above:  One day I was on a
pier at the ocean.  There was a fisherman sitting on the edge of the pier,
with a fish he had caught  lying on the pavement about 3 feet from him.  It
was a striped bass, probably about 15 inches long.  A gull was standing
about 10 or 12 feet away.  Every time the fisherman looked away, the gull
would inch a little closer, and every time the fisherman turned toward the
gull, it would stop and look away, like the fisherman was the last thing on
its mind.  After a few minutes the gull got to a point about 6 feet behind
the man.  When the man turned away, the gull made its move, ran over,
grabbed the fish and took off with it.  It didn't have a very good grip, and
dropped the fish in the water, but then circled back, picked it up and flew
off with it.

Paul M.