Pyrgophorus is a genus of very small freshwater snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s with a gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

 and an operculum

Operculum (gastropod)

The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...

, aquatic

Aquatic animal

An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

 gastropod molluscs in the family Hydrobiidae

Hydrobiidae

Hydrobiidae, common name mud snails, is a large cosmopolitan taxonomic family of very small freshwater snails and brackish water snails that have an operculum, aquatic gastropod mollusks in the clade Littorinimorpha.- Distribution :...

. They

Species

Species in the genus Pyrgophorus include:

• Pyrgophorus parvulus (Guilding, 1828)
• Pyrgophorus platyrachis
  Pyrgophorus platyrachis
  Pyrgophorus platyrachis is a species of very small aquatic snail, an operculate gastropod mollusk in the family Hydrobiidae....
  
  F. G. Thompson, 1968 - serrate crownsnail
• Pyrgophorus spinosus
  Pyrgophorus spinosus
  Pyrgophorus spinosus, common name the spiny crownsnail, is a species of very small freshwater snail with a gill and an operculum, an aquatic gastropod mollusk in the family Hydrobiidae.- Taxonomy :...
  
  (Call & Pilsbry, 1886) - spiny crownsnail
• Pyrgophorus coronatus
  Pyrgophorus coronatus
  Pyrgophorus coronatus is a species of very small aquatic snail, an operculate gastropod mollusk in the family Hydrobiidae.-Distribution:In Nicaragua, it is found in abundance in the volcanic crater lakes Xiloa and Apoyo -References:...
  
  (Pfeiffer, 1840)

External links

http://www.gaianicaragua.org/McCraryRBT.pdf
Comparison of gastropod mollusc (Apogastropoda: Hydrobiidae)
habitats in two crater lakes in Nicaragua,
Jeffrey K. McCrary, Henry Madsen, Liza González, Inti Luna, & Lorenzo J. López. Rev. Biol. Trop. 56:113-120 (2008)