Aselenizone (from theGreek "selene" meaning "moon", and "zone" meaning "girdle") is an anatomical structure that exists in theshells of some families of livingsea snails: theslit shells, thelittle slit shells and theabalone, which aremarinegastropodmollusks from ancientlineages.

It is a spiral band of crescentric growth lines or threads (lunulae) on the shell surface due to the semicircular end of a notch or slit on the outer lip.^[1]

A structure of the same type exists in several fossil groups of mollusks, including all the fossil families of slit shells, as well as three superfamilies of what may have been gastropods, but may possibly have beenmonoplacophorans orparagastropods instead.

The function of the holes and slits in living sea snails is to allow forexhalant water circulation, which is important forrespiration and other functions. It has also been suggested that the selenizone may serve to reinforce the shell against catastrophic breakage during predation attempts.^[2]

A new shell-morphological term 'sutsel' has been introduced by Dr. Geiger for the area between theSUTure and the SELenizone.^[3]

In thePleurotomariidae (the slit shells), and theScissurellidae (the little slit shells), the selenizone is a groove on the periphery of the shell which forms as the shell grows by the gradual sealing up of a slit that extends back from the edge of theaperture of the shell.

In theHaliotidae, theabalone, the selenizone takes the form of a series of holes in the shell (reaching back from the leading growth edge of the aperture) which form as the shell grows. The older holes are gradually sealed up as the shell grows and new holes form. Each abalone species has a typical number of holes in the selenizone which remain open, and this feature is diagnostic for the individual species.

A selenizone is also present in several superfamilies of fossil mollusks. Not surprisingly it is seen in the numerous fossil gastropod families within the Pleurotomarioidae. The selenizone also exists in theBellerophontoidea, which may be gastropods ormonoplacophorans; in theEuomphaloidea, which are probably gastropods, but may be monoplacophorans; and in theMacluritoidea, which are eitherarchaeogastropods orParagastropoda Linsley & Kier, 1984, (paragastropods are a group of mollusks that superficially resemble gastropods, but wereuntorted).^[4]

A list of superfamilies and families (both living and fossil) which show this structure, based on the taxonomy of Bouchetet al 2005.^[5] The taxa that are entirely extinct are marked with a dagger † :

Extant taxa:

• Haliotoidea
  • Haliotidae
• Pleurotomarioidea
  • Pleurotomariidae
• Scissurelloidea
  • Anatomidae
  • Scissurellidae

Fossil taxa:

Gastropods

• Haliotoidea
  • †Temnotropidae
• Pleurotomariacea
  • † FamilyCatantostomatidae
  • † FamilyKittlidiscidae
  • † FamilyPhymatopleuridae
  • † FamilyPolytremariidae
  • † FamilyPortlockiellidae
  • † FamilyRhaphischismatidae
  • † FamilyTrochotomidae
  • † FamilyZygitidae

Gastropods or monoplacophorans or paragastropods

• †Bellerophontoidea
  • †Bellerophontidae
  • †Bucanellidae
  • †Bucaniidae
  • †Euphemitidae
  • †Pterothecidae
  • †Sinuitidae
  • †Tremanotidae
  • †Tropidodiscidae
• †Euomphaloidea
  • †Euomphalidae
  • †Helicotomidae
  • †Lesueurillidae
  • †Omphalocirridae
  • †Omphalotrochidae
• †Macluritoidea
  • †Macluritidae

• Selenizone mentioned here
• Paper on the selenizone and breakage in fossil shells
• Information on paragastropods here

• Mollusc shells
• Gastropod anatomy