Clionoidea is a taxonomic superfamily

Taxonomic rank

In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...

 of sea slugs, specifically naked (i.e. unshelled) pteropods, marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 opisthobranch gastropod mollusks in the clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Gymnosomata. They are sometimes called "sea angels" along with the other superfamily in the Gymnosomata.

Taxonomy

In the new taxonomy of Bouchet & Rocroi

Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)

The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...

 (2005), the superfamily Clionoidea consists of the following families :

• Family Clionidae
  Clionidae
  Clionidae is a family of sea angels, which are a group of pelagic marine gastropods.They are shaped a little like angels, and have flapping "wings", hence their common name. They are gelatinous, mostly transparent pteropods, and they only have shells in their embryonic stage...
  
  
• Family Cliopsidae
  Cliopsidae
  Cliopsidae, common name sea angels, are a taxonomic family of small free-swimming sea slugs, pelagic marine opisthobranch gastropod mollusks in the order Opisthobranchia....
  
  
• Family Notobranchaeidae
  Notobranchaeidae
  Notobranchaeidae is a taxonomic family of floating sea slugs or "sea angels", pelagic marine opisthobranch gastropod mollusks.The small pelagic snails of this family have no shell...
  
  
• Family Pneumodermatidae
  Pneumodermatidae
  Pneumodermatidae is a family of sea angels, or small floating predatory sea snails or sea slugs. They are pelagic marine heterobranch opisthobranch gastropod mollusks in the clade Gymnosomata....