From @Architeuthoceras I understand that ammonites grew new shell chambers and "moved" forward into the new growth, keeping and closing off the old housing. Somehow I had it in my head that this was true of the nautilus as well because they also have chambers and a siphuncle running through the shell. I have seen different sized nautilus but it did not occur to me until watching @gjbarord 's video (download required, great resolution - S3 in his new publication) that this is definitely not the way the shells grow in the nautilus as they remain the same shape throughout the animals life. I assume that snails do something similar but since they don't shed like a crab how do the shells grow to accommodate the growing animal?