tooth is trapezoidal. The lateral teeth have few to numerous cusps. The marginal teeth are usually with numerous cusps. The species usually have both male and female individuals, but very rarely reproduction may be parthenogenic, caused by internal fertilization. The females lay eggs in single capsules on the leaves or stems of water plants. But sometimes they produce eggs that are hatched within the pallial gonoduct of the body, and in these cases the young are born alive. Species that inhabit estuaries sometimes produce veliger larvae. The fossil record of this family extends back to the Early Carboniferous. Ecology Habitat Most species of this family live in freshwater (lakes, ponds, rivers, streams), but some are found in brackish water or at the borders between freshwater and brackish water. A few occur in marine environments on sandy or muddy bottoms between algae and sea grass. Feeding habits These small snails feed on algae, diatoms and detritus. Taxonomy This is the largest family within the superfamily Truncatelloidea. At one time or another some 400 genera have been assigned to this family, and probably more than 1,000 species. The Hydrobiidae family is the most widespread and diverse family of freshwater molluscs in the world, occupying a range of habitats from streams and estuarine creeks to alpine bogs. This family was originally named by Troschel in 1857, as the group Hydrobiae. Troschel was not certain of their rank, and he placed them in the Taenioglossata: Ctenobranchiata between the Lithoglyphi and Ancyloti. Over the years there have been numerous attempts to give an adequate and more finely divided classification. Thiele (1925, 1929 and 1931) set up the most comprehensive classification, with a review of the family at generic level. Notable works about taxonomy of Hydrobiidae includes works by Radoman, for example Radoman (1983). The classification, as specified by Kabat and Hershler (1993), does not form a monophyletic group, and was in need of revision. A study by Wilke et al. (2001) using molecular data from COI (Cytochrome c oxidase subunit I) and 18S genes has resulted in a new tentative set of subfamilies: Hydrobiinae, Pseudamnicolinae, Nymphophilinae, Islamiinae and Horatiinae. 2005 taxonomy The following subfamilies are recognized in the gastropod taxonomy of Bouchet & Rocroi from 2005: Hydrobiinae Stimpson, 1865 – synonyms: Paludestrinidae Newton, 1891; Pyrgorientaliinae Radoman, 1977; Pseudocaspiidae Sitnikova & Starobogatov, 1983 Belgrandiinae de Stefani, 1877 – synonyms: Horatiini D. W. Taylor, 1966; Sadlerianinae Radoman,