Like people, different individuals of a mollusc species may grow at different rates, and reach different sizes before the growth hormones taper off. Also, some molluscs, like bivalves, tun shells, moon snails, crown conchs, cone shells, limpets, and many others continue to grow as long as they live, while others, like cowries, egg shells, conchs, and pelican-foot shells tend to reach a maximum size, and then stop adding to overall shell size (though they may continue to add to the thickness and weight of their shells). As Bill already mentioned, a conch shell is mature when it's lip becomes thick and solid, whether it is 6 inches long or 12 inches long at that time. And it is still immature as long as the lip is thin and fragile, regardless of it's size. "Growth rings" cannot be used to tell the age of a shell, except in certain cases. Bivalves which live in shallow water, in climates which fluctuate between frigid and warm, may have true annular growth rings, since they grow rather quickly during the warmer months, and virtually stop growing during the winter. Bivalves which live where the temperature is uniform year round (either constantly warm or constantly cold) tend to grow at a more uniform rate. This includes deep water species in virtually any climate. Such species may still experience periods of faster growth and periods of slower growth, based on fluctuations in food supply or other factors, and their shells may show corresponding growth lines, but these lines are less likely to correspond to years of age on a 1-to-1 basis. All of the above also pertains to gastropods (snail-type molluscs), but in gastropods there is another twist (no pun intended). Later whorls of the shell cover over the earlier whorls, so that earlier growth lines or varices are no longer visible. In some forms, like Cassis and Cymatium, evidence of earlier varices may be seen in the spire. In many other forms, this is not the case. Anyway, again, in such species there is no guarantee that each period of growth corresponds to a year of age. Paul M.