Most collectors like their specimens to be as close to perfect as possible.  However, the sea is  very harsh place to live in practice, and the shell's surface is subjected to a wide variety of hazards such as attacks by other organisms, (some of which wish to make a meal of the mollusc inside, and others who simply want a place to settle down and attach to (called a "substrate")), variable Calcium carbonate concentration (CaCO3 - the substance molluscs and other organisms such as star fish and sea urchins use to make their shells: if it is too low, the shell is dissolved, or is at best very difficult to capture from the water!),  high acidity in some fresh and brackish waters (acidity dissolves the shell if it is exposed - NOT a great thing for a mollusc trying to survive intact!), and abrasive sand and silt.  The Periostracum, which our "Mollusc of the moment" for today displays so well, is designed to protect the shell from many of these dangers.  It is made of a substance called "conchiolin", which is a tough, outer layer of protein that is secreted from the leading edge of the mantle, and clings tightly to the shell, often sealing it off completely, effectively preventing the shell from coming into contact with the water which could dissolve it,  the sand or silt which could erode it (something shell collectors really appreciate!!), and organisms that wish to find a home or a meal!  (although its protection from the later is often minimal!!  After all, if a predator can bore through a shell, it can usually handle a bit of concholin!!).  The protein Concholin is made of, is similar to the material our own bodies use to make hair and fingernails.