Shells appearing in my tank!!

Over the past few weeks, I have notices shells appearing on the bed of my tank... the shells were not in the sand when it was put into the tank and I have been told that this sand is 'man made'.



Here's a few images of the biggest shell I have found so far



http://ift.tt/1OqWuoA

http://ift.tt/1yDSjE2



Anybody have any idea what they are/how they got there??