Pecos Assiminea (Assiminea pecos) is a tiny (shell length up to 2.2 mm) thermal spring snail with rudimentary gills. They are the most inland member of their group and are now only found along the Lost River in Bitter Lake National Wildlife Refuge in New Mexico and in Diamond Y Draw in Texas. They inhabit aquifer-fed spring systems in desert grasslands with abundant karst (limestone land formations) topography.