Helcionellacea

Helcionellacea
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
Subclass:	Prosobranchia
Order:	Bellerophontida
Superfamily:	Helcionellacea Wenz, 1938

The Helcionellacea is one of two taxonomic superfamilies of extinct primitive marine molluscs within the order Bellerophontida. This superfamily is thought to be the more primitive of the two superfamilies, the other being the Bellerophontacea. These molluscs are thought to be sea snails, marine gastropod mollusks, rather than monoplacophorans.

Helcionellaceans are characterised by cap-shaped to bellerophontiform shells; commonly with strong rugae, (transverse plications) clearly defined on both the interior and exterior of the shell; with a septum or septa partitioning off the apex, but lacking the anal emargination that is characteristic of the Bellerophontacea.

The Helcionellacea includes two families according to the Treatise,1960, the Helcionellidae, which is the more primitive, and the Coreospiridae, which is the more derived.

The Helconellidae (Wenz, 1938) represented by the genus Helcionella (Grabau & Shimer, 1909) more closely resembles the monoplacophoran ancestor, and is characterized by elongate cap-shaped shells.

The Coreospiridae (Knight, 1947) which includes Coreospira (Satto, 1936) Cycloholcus (Knight, 1947) and Latouchella (Cobbold, 1921) are more bellerophontiform (shaped like a bellerophont). Latauchella has the least amount of apical coiling, and more closely resembles species of Helcionella. Coreospira is tighly coiled while Cycloholcus is open coiled. Both bear some resemblance to the genus Cloudia in the Cyrtolitidae, Bellerophontacea.

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Wikipedia