CENTRIC DIATOMS FROM THE MIOCENE DEPOSITS IN THE BAIKAL RIFT-ZONE (TUNKA BASIN)

New species of centric diatoms from the genera Cyclotella Kutz, Stepha nodiscus Ehr. and variety from the genus Aulacoseira Thw. have been de scribed from the Miocene deposits in the Baikal Rift Zone (Tunka basin , BH-73). Cyclotella tuncaica sp. nov. has strongly tangentially wavy valves of 14-40 mu m in diameter. In appearance it is similar to C. di stinguenda Hust. but differs from it in having an oval central part an d strong tangential waving, in closely-arranged marginal tultoportulae with satellite pover and in structupe of rimoportula. According to da ta of scanning electron microscopy, it is not similar to the baikalian species C. minuta (Skv.) Antipova, but under the optical microscope C . tuncaica may be mistaken for C. minuta. Stephanodiscus tuncaensis sp . nov. has valves of 15-24 mu m in diameter and resembles S. bellus va r. minor Churs. et Log. in having a single central process tultoportul ae with three satellite pores but is distinguished by strong concentri c waving, character of arrangement of areolae in the central part, and their lack of well-expressed radial rows. The new species is not simi lar to the fossil baikalian species S. grandis Churs, et Log., S. carc oneiformis Churs. et Log., S. flabellatus Churs, et Log. Aulacoseira p raegranulata var. tuncaica var. nov. has valves of 5-7 mu m in diamete r and 4-17 mu m in height. According to a number of characters, it may be diagnosed as a polymorph, having several varieties, species A. pra egranulata (Jouse) Simonsen. New variety differs from A. praegranulata (Jouse) Simonsen var. praegranulata and var. praeislandica (Jouse) Mo iss. by the shape of lincking spines, distribution pattern of areolae and their structure; from var. praegranulata it also differs by a lowe r mantle of valve. From A. baicalensis (K. Mayer) Simonsen the new var iety differs by the shape of lincking spines, the shape and structure of areolae. In addition, valves of several representatives of the genu s Aulacoseira have been described morphologically to demonstrate in to tal a great diversity of this genus in the matter under study. Some of them have parameters similar to those of A. baicalensis.