Empty shells of land snails usually accumulate soil and all sorts of debris in them, especially if they have wide apertures. One can dislodge most of what is inside the bodywhorl of a shell by squirting water inside, by gently tapping on the shell or by inserting something long and curved with a blunt tip deep into the shell. Often, the soil that comes out of a large shell will have tiny snail shells in it. Therefore, it is a good idea to sort thru that soil before discarding it away.
Yesterday afternoon I examined some land snail shells (in the family Hygromiidae, to be specific) that had been collected near a beach. A small amount of beach sand came out of the shells, which I collected in a small dish. And before tossing it away, I scanned it under the stereomicroscope. Among the tiny sand grains was this tiny foraminiferan shell almost exactly a millimeter wide.
Foraminifera, called forams by those who study them, are mostly microscopic and exclusively marine organisms in the kingdom Protista.
In the 1950s, the Foraminifera, along with other microscopic organisms ("protozoa"), were considered to be animals. For that reason, there is a chapter about them in Invertebrate Fossils by Moore et al1. Many foraminiferan shells (tests) look quite like snail shells. And like snail shells, most foraminiferan shells are also made of CaCO3.
Some forams with snail-like shells. Drawings from Moore et al1.
1. Moore, Lalicker & Fischer. 1952. Invertebrate Fossils. McGraw Hill.