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.[1]
| Helcionelloida
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|---|---|
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| Latouchella costata | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Subphylum: | Conchifera |
| Class: | †Helcionelloida Peel, 1991 |
| Orders | |
and see text | |
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.
The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005[2] categorizes members of this taxon within the Paleozoic molluscs of uncertain systematic position, but that taxonomy to not use the name Helcionelloida.
Taxonomy of helcionelloid according to the opinion of P. Yu. Parkhaev:[3][4]
Gastropoda Cuvier, 1797
Subclass Archaeobranchia Parkhaev, 2001
Subclass Divasibranchia Minichev & Starobogatov, 1975
Subclass Dextrobranchia Minichev & Starobogatov, 1975