Shells are mainly divided into two big classes:

· Gastropoda, with very varied shells built like those of the snail. Of these I have found a great variety of shells belonging to 42 families.

· Bivalves, which have two mostly symmetrical parts held together by the muscle of the animal inside. Of these shells I have found species belonging to 32 families.

· Of a third small class, the Scaphopoda or tusk shells, I have only found species belonging to one family.