Turritella Agate is the popular name used for a brown, translucent, fossiliferous agate found in the Green River Formation of Wyoming. It is easily recognizable because it contains large fossil snails that stand out in a white-to-tan color in contrast with the brownish agate.
This organic gem material was incorrectly named decades ago when it was thought that the spiral-shaped gastropod (snail) fossils within the stone were members of the marine Turritella genus. Actually, the fossils are of the freshwater snail, Elimia tenera, a member of the Pleuroceridae family.
"Before the correct name was realized and widely published, the gem material became quite popular and the name "Turritella" went wild in lapidary magazines, gem, mineral, and fossil books, catalogs, and exhibits. Today it is typically seen without corrective note in all of those sources, along with websites, online auctions, and computer software. Only a fraction of the people who have collected the material, cut it into cabochons, sold it, bought it, or worn it in jewelry have any knowledge that Elimia is a more appropriate name." [Geology.com]