Some burrowing bivalves have markedly different sculpture in juveniles and
adults. Imagine yourself as a tiny juvenile clam: To you, sand grains are
the size of bricks. So young burrowers commonly have relatively thick
ridges to act as ratchets to move themselves against these "huge" sand
grains. The adult clam doesn't need the sculpture; in fact, it makes it
more difficult to glide through the sediment. Their shells are smooth...
except at the umbo, where the juvenile stage of growth may still be
preserved.
 
I'd like to know more about why juvenile shells are shaped differently from
adult shells of other kinds of mollusks.
 
Andrew K. Rindsberg
Geological Survey of Alabama