Class Gastropoda: Subclass Pulmonata: Order Basommatophora.
A cosmopolitan family of limpet-like snails, without shell coiling. Three Australian species are described in two genera, and the distribution is widespread. Ancylids occur in a variety of freshwaters but are rarely abundant and often overlooked by collectors because of their small size and habit of clinging to the underside of rocks and vegetation.
There is a single Tasmanian species in family Planorbidae that is superficially ancylid-like, but it has a vestige of coiling at the apex of the shell. Otherwise, identification of ancylids to family level should be trouble-free.
The Australian fauna comprises two genera, Ferrisia and Stimulator . The former contains two species, separated on the relative height and size of the shell, and variation in presence of pigment. The latter is known from a single species in far north-west Western Australia. The posterior shell margin may develop a septum or shelf - a horizontal outgrowth that may deverlop enough to resemble a cap on the shell. Although this has been treated as of specific significance, the development appears to be ecologically induced and not taxonomically significant.
Smith, B.J. (1996) Identification keys to the Families and Genera of Bivalve and Gastropod Molluscs found in Australian Inland Waters . Cooperative Research Centre for Freshwater Ecology Identification Guide No. 6., Albury, NSW.