中文摘要：

在对大兴安岭沼泽硅藻进行分类学研究的过程中，观察到桥弯藻科中国新记录8个分类单位，分别为平截盘状藻Placoneis explanata（Hust．）Lange-Bertalot、类冰川盘状藻Placoneis interglacis（Hust．）Cox、近北极桥弯藻Cymbella subarctica Krammer、佛罗伦萨弯肋藻Cymbopleura florentina（Grun．）Krammer、新异肋弯肋藻Cymbopleura neoheteropleura Krammer、近尖头弯肋藻Cymbopleura subcuspidata（Krammerl Kram—mer、近喙头弯肋藻Cymbopleura subrostrata（c1．）Krammer、埃尔金内丝藻Encyonema elginense（Krammer）Mann。异极藻科中国新记录7个分类单位，分别为英吉利异极藻GomphonemaanglicumEhrenb．、远距异极藻Gomphonema distans（C1-Eul．）Lange—Bertalot ＆ Reichardt、肯特异极藻Gomphonema khentiiense Metzeltin，Lange—Bertalot ＆ Nergui、李氏异极藻Gomphonema liyanlingae Metzeltin ＆ Lange．Bertalot、铲形异极藻Gomphonema pala Reichardt、假弱小异极藻GomphonemapseudopusiIlumReichardt、箭形异极藻Gompho。nemasagittaSchumann。文章对这些种类的形态特征进行了观察，对这些种类与相似种类之间的形态差异进行了比较研究，并初步探讨了这些种类的生态分布特点。对盘状藻属Placoneis Mereschkowsky、内丝藻属Encyonema Ktitzing和弯肋藻属Cymbopleura（Krammer）Krammer的分类特征进行了详细的描述。

英文摘要：

Cymbellaceae and Gomphonemataceae were established in 1833 by Greville and 1844 by Kiitzing, respec- tively. But in preliminary study, both families were put into Diatomaceae by Smith 1872 or into Naviculaceae （subfam- ily Gomphocymbelloideae） by Hustedt 1930. Until 1960--1970s, Patrick ＆ Reimer gave these two families the same status as Naviculaceae, belong to Biraphidinales. Round, et al. established a new order - Cymbellales in 1990, based on the structure of diatom valves in SEM, including Rhoicospheniaceae, Anomoeoneidaceae, Cymbellaceae and Gompho- nemataceae. Cymbellaceae include： Placoneis Mereschkowsky, Cymbella Agardh, Brebissonia Grun., Encyonema Kiatzing and Gomphocymbella MSller. Krammer seperated Cymbopleura subgenus from Cymbella, Cymbopleura （Krammer） Krammer were established in 1999. Gomphonema Ehrenb., Didymosphenia Schmidt, Gomphoneis CI., Reimeria Kociolek ＆ Stoermer and Gomphopleura Reichelt ＆ Temp6re belonged to Gomphonemataceae. Great Xing＇an Mts. locats in northeast of China with a large area of wetland. During the investigation of diatoms from Great Xing＇an Mts., species that belonged to Cymbellaceae and Gomphonemataceae preferred habitats with pH values ranging from 6 to 9. In total, we observed that 5 genera and 44 taxa belong to Cymbellaceae and 4 genera and 24 taxa belong to Gomphonemataceae. The Chinese name for Placoneis was firstly used among these genera. The detailed description for the genera Placoneis Mereschkowsky, Encyonema Ktitzing and Cymbopleura （Krammer） Krammer were given. For many years, Placoneis was within Navicula sect - Lineolatae, but it is much closely related to Cymbella and Gomphonema with a single, large elaborate plastid which is divided into two X-shaped plates lying against each valve, connected by a broad column which contains a large flat pyrenoid; Striae uniseriate, containing small round poroids closed by volate occlusions; Internal central raphe endings hooked towards the primary side of the valve; External cen- tral endings sli