This beautiful one is called Paper Nautilus. Nautiluses are actually not gastropods. They are Cephalopods, same class as those of squids and octopus. The shells of nautilus are most apparent as equiangular spirals.
It is extremly thin and light, as its name implies. One can see that its so thin that its shell is trunslucent. It is so light that if one drops it in midair, it doesn't fall straight-away like a rock but rather like a crunched paper ball. A light squeez by the fingers can break the shell. It measures 6.5 cm. It is bought on ebay.com around 2002. Very very beautiful, angle-like.