"Most commonly they are formed by outward growth of the surface from a central nucleus, although this is not the only way they can form. Some nodules derive directly from the weathering parent rocks."
Could it have been that the snail formed the nucleus around which the concretion was formed ? Or was it included during the formation of the concretion ? I have yet to fully understand the 'mythical' nature of these concretions ? What problem do they present that cannot or could not be solved by convential physics ? What do you mean by the mud-flat ?
http://www-aghort.massey.ac.nz/departs/soilsc/cybsoil/text/pans.htm
"Nodules of calcium carbonate form in soils of very low rainfall areas. So their genesis is quite different, being dependent more on a lack of water than a surplus. Their growth can be crystalline, rather than smooth like the other concretions discussed above."
Or
Identifiable Secondary Carbonates
"Identifiable secondary carbonates is a term used in the definitions of a number of taxa. It refers to translocated authigenic calcium carbonate that has been precipitated in place from the soil solution rather than inherited from a soil parent material such as a calcareous loess or till."
"Identifiable secondary carbonates may either disrupt the soil structure or fabric to form masses, nodules, concretions, or spheroidal aggregates (white eyes) that are soft and powdery when dry; or it may be present as coatings in pores, on structural faces, or on the undersides of rock or pararock fragments. If present as coatings, it covers a significant part of the surfaces. Commonly, it coats all of them to a thickness of 1 mm or more; but if little calcium carbonate is present in the soil, the surfaces may be only partially coated. The coatings must be thick enough to be visible when moist. In some horizons with much calcium carbonate the entire horizon is colored by secondary carbonates. "