Nearly 400 species of hypotrichs have been de- scribed during the last 100 years, but their classification does not reflect their most probable phylogenetic relationships. Recent developments in our understanding of structural and morpho- genetic variation now allow for critical evaluation of descriptions of species, explicit definitions of genera, and assignment of genera to families. Following a historical review of classification schemes used previously, morphogenetic bases are given for revisions at the family level, based on a standardized terminology of struc- tural features. The 6 families Urostylidae (9 genera), Psilo- trichidae (3 genera), Holostichidae (7 genera), Spirofilidae (7 genera), Oxytrirhidae (14 genera), and Euplotidae (1 1 genera) are now recognized. The suborders erected by Faurk-Fremiet in 1961 are not recognized in this revision. Revision at the genus and species levels is based on the known limits of structural variation as well as decisions relative to synonyms and homonyms. Some described species are excluded because they are insufficiently described or interpretable as dis- torted specimens of unknown systematic position. New genera Paraurostyla n. g. and Pseudourostyla n. g. are proposed. The new names Spiratella and Psammornitra are proposed as replace- ment names for Spiretta and Micromitra which must fall as junior homonyms. All families and genera are diagnosed and illustrated, and synonymies are listed for each species. The revision recognizes 310 species distributed among 51 genera and 6 families.
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