Helcionelloida
Helcionelloida is an extinct group of ancient molluscs (phylum Mollusca). These are the oldest known conchiferan molluscs, that is, they had a mineralised shell. Some members of this class were mistaken for Monoplacophorans. The class was erected by Peel in 1991. Anatomy. These animals were untorted and they had a coiled, cone-shaped shell. The majority of species were small (shells being about two millimeters in length and or diameter). Modern reconstructions depict them as resembling snails. The shells of some genera, particularly "Yochelcionella", also possessed a "snorkel"-like opening which was most likely used for breathing. 2005 taxonomy. The  taxonomy of the Gastropoda by Bouchet & Rocroi, 2005 categorizes members of this taxon within the Paleozoic molluscs of uncertain systematic position, but that taxonomy to not use the name Helcionelloida. 2006-2007 taxonomy. Taxonomy of helcionelloid according to the opinion of P. Yu. Parkhaev: Gastropoda Cuvier, 1797 Subclass Archaeobranchia Parkhaev, 2001 Subclass Divasibranchia Minichev & Starobogatov, 1975 Subclass Dextrobranchia Minichev & Starobogatov, 1975