Redescription of Psilotricha acuminata Stein, 1859 and revisions of the genera Psilotricha and Urospinula (Ciliophora, Hypotrichida)

Psilotricha acuminata was described by Stein in 1859 as the type species of the ciliate genus Psilotricha Stein, 1859. The ciliate has rarely been fou nd since, and its infraciliature has never been described with the aid of s ilver-impregnation techniques. We have found P. acuminata Stein. 1859 in so il samples from upland grassland in Scotland (U.K.). Living and healthy org anisms of P. acuminata are oblong in outline, and dorso-ventrally compresse d. They closely resemble ciliates of the genus Euplotes. The main morpholog ical features used for identification of P. acuminata are the very long and sparse cirri. and the two macronuclei. When the ciliate crawls, the cirri appear stiff and directed backwards. Specimens observed from the ventral si de have a protruding anterior end, a rounded or acuminate posterior pole, a nd a "beak-like" projection to the left side of the posterior end. The cili ate shares characters with the Euplotidae (body shape and reduced ciliature ) and with the Oxytrichidae (marginal rows, macronuclei, reduced number of transverse cirri). Because the arrangement of the silver-impregnated infrac iliature was unknown, and as the only description of the ciliate was that o f Stein (1859a, b). the genus Psilotricha became confused with other hypotr ich genera, especially Urospinula Corliss, 1960. Here we provide a full red escription of P. acuminata based on living and silver-impregnated specimens , and a revision of the genera Psilotricha Stein. 1859 and Urospinula Corli ss. 1960. We resurrect the genus Urospinula, and give an emended diagnosis for both genera. The species now included within the genus Psilolricha are P. acuminata Stein, 1859 (type species), Psilotricha viridis (Penard, 1922) Kahl, 1932: and Psilotricha geleii (Gelei, 1954) Stiller, 1974. Psilotrich a viridis sensu Kahl, 1932 and Psilotricha dragescoi Groliere. 1975 are con sidered incertae sedis.