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Four new monoraphid diatoms are examined and described from a high-altitude (> 3000 m) region of western Sichuan, China. The descriptions are based on light and scanning electron microscopy observations, and the four new species are compared with similar taxa worldwide. Achnanthidium sichuanense sp. nov. has small, lanceolate-elliptical valves, with rounded apices. The outline and central area differentiate it from similar species. The unique characteristic of Eucocconeis undulatum sp. nov. is that it has valves that are undulate along the apical axis, forming an “H”-shaped area on both raphe and rapheless valves as seen in light microscope (LM). Platessa mugecuonesis sp. nov. and Platessa lanceolata sp. nov. are two small species, whose valve outlines and central areas make them different from other species in this genus. A. sichuanense sp. nov. and E. undulatum sp. nov. are discussed in terms of the relationship between “Achnanthidium & Psammothidium” and “Eucocconeis & Psammothidium”, as these genera are difficult to separate according to morphological characteristics.

Приводятся результаты анализа литературных и оригинальных данных по экологии и географическому распространению 5 видов рода Eucocconeis P.T. Cleve во флоре водорослей Западной Сибири. Для каждого вида приводятся сведения по отношению к галобности, активной реакции воды, сапробности и географическому распространению.

The results of the analysis of literature and original data on ecology and geographical distribution of 5 Eucocconeis P.T. Cleve species in the algal flora of West Siberia are given. Information relevant to halobility, active water reaction, saprogenity and geographical distribution is presented for each of species., Приводятся результаты анализа литературных и оригинальных данных по экологии и географическому распространению 5 видов рода Eucocconeis P.T. Cleve во флоре водорослей Западной Сибири. Для каждого вида приводятся сведения по отношению к галобности, активной реакции воды, сапробности и географическому распространению.

A new diatom species, Eucocconeis americana, is described from eastern North America. The new species differs from previously reported representatives of Eucocconeis Cleve by relatively flat valves and the characteristic shape of the central areas in both raphe and rapheless valves. We present details on the frustule structure and size variation, and compare the new species with other representatives of Eucocconeis. This new species was found in rivers and streams from the eastern USA characterized by low conductivity and pH.

Summary This contribution to the revision of the genus Achnanthes sensu lato concerns a section of 38 apparently closely related taxa. Neither do they fit into Achnanthes sensu stricto with the type species A. adnata (syn. A. brevipes ) nor into Achnanthidium with type species A. microcephalum (syn. A. minutissima ). They also do not conform with the morphologic pattern of Eucocconeis or of the recently described Psammothidium . The proposed new genus Achnantheiopsis is characterized by a unique complex of distinct features. These are represented in particular by the most prominent species respectively species clusters around A. lanceolata and A. delicatula .