From Ohio History Central
Snails (Class Gastropoda), or gastropods, are familiar to most everyone who has collected shells along a seashore or observed a terrestrial snail in the garden. They have been a very successful, diverse, and long-ranging group, having originated in the Cambrian. Shells of snails are well-represented in Ordovician through Pennsylvanian marine rocks and freshwater species are known from Pennsylvanian and Permian rocks and from deposits of the Pleistocene Ice Age. Most specimens are small, less than an inch long.
Snails are characterized by a single shell that is coiled, either in a flat plane (planisprial) or in a cone shape (conispiral). They have a foot that is used to crawl across the bottom in search of food and a mouth with a rasp-like tongue. Some modern species are predatory and are noted for boring circular holes in clam shells in order to get to the soft parts.
Many Ordovician and Silurian snail shells are internal molds of the shell (steinkerns) as the original calcium carbonate shell has been removed. These specimens are sometimes difficult to identify, as the external shell ornamentation is absent.