This volume of the Joint Soviet-Mongolian Biological Expedition contains papers by Soviet and Mongolian paleontologists and geologists. The contributions are thematically divided into two principal sections. The first section is devoted to paleontological and biostratigraphic studies of Mesozoic and Cenozoic continental deposits in Mongolia, while the second addresses investigations of marine Paleozoic strata. A series of papers in the first section describe fossils of various organisms (vertebrates, invertebrates, and plants) discovered by the South Gobi Detachment of the Joint Soviet-Mongolian Expedition during 1970-1973 at the Late Paleocene localities of Naran Bulak and Tsagan Khushu in the Ikh Bogd Basin of southwestern Mongolia (Naran Bulak Formation). The volume opens with a paper by V. Yu. Reshetov describing Coryphodon tsaganensis, a new species of primitive pantodont ungulate discovered for the first time in Mongolia (at Tsagan Khushu). A paper by the Mongolian paleontologist D. Dashzeveg is devoted to mesonychid carnivorous condylarths from the Naran Bulak and Khaychin Ula II localities. M. B. Efimov describes a fragment of a crocodilian skull recovered for the first time from deposits of the Naran Bulak Formation at Tsagan Khushu. V. B. Sukhanov and P. Narmandakh investigated two new turtle species from Paleocene deposits of southern Mongolia (the Naran Bulak and Dakhin formations): Mongolemys tatarinovi and M. reshetovi. It should be noted that representatives of the genus Mongolemys had previously been known only from the Late Cretaceous. The same authors also described a new genus and species of turtle from the Naran Bulak Formation, Pseudochrygemys gobiensis, the earliest known member of the family Emydidae in Central Asia. D. Badamgarav and N. V. Tolstikova's paper is devoted to opercula of Bythyniidae gastropods from Naran Bulak and the Early Paleogene deposits of the Zaysan Basin in eastern Kazakhstan. N. Hand described ostracods from the Barun Goyot Formation (Upper Cretaceous) and the Naran Bulak Formation (Upper Paleocene) of southern Mongolia. E. K. Trusova and D. Badamgarav published for the first time a conchostracan assemblage from Naran Bulak, a group not previously recorded from Cenozoic deposits. L. I. Fodianova and D. Badamgarav described remains of four fossil plants (three angiosperms and one conifer) discovered for the first time in lacustrine-alluvial clay deposits from the middle part of the Naran Bulak Formation at Naran Bulak. It should be emphasized that this represents the first discovery of an impression flora in Mongolia's Lower Tertiary deposits. Thus, the new materials presented in the volume substantially supplement previous data on the flora and fauna of the Naran Bulak Formation and make possible a comprehensive assessment of its age. The published data will in future be supplemented by new discoveries and will serve as the basis for a monographic study of the Naran Bulak fauna. Closely related in subject matter is the paper by V. I. Zhegalo and N. S. Shevtsova revising the famous Upper Paleocene section of Upper Gashato (Khashaat). The paper presents new geological information concerning the locality. The presence of basalts within the distribution area of the “Gashato Formation” made it possible to determine its absolute age. N. S. Shevtsova also described from this locality an indeterminate primitive rodent, Gomphos elkema. The remaining papers in this section concern various vertebrate and invertebrate faunas and their biostratigraphy in Mongolia's Mesozoic and Cenozoic continental deposits. I. A. Dubrovo described two gomphothere proboscideans from the Great Lakes Basin: Tetralophodon sinensis (Koken) from Late Miocene deposits and Anancus sinensis (Hopwood) from Late Pliocene deposits. N. M. Yanovskaya described a new brontothere species, Epimanteceras omplus, from the lower part of the Lower Oligocene at Ergilin-Tso in Mongolia. D. Dashzeveg reported new information on entelodonts from the Early Oligocene of eastern Mongolia and described a new species, Eomelodon trofimovi. In a lengthy paper, E. N. Kurochkin described and analyzed more than forty fossil remains of shorebirds and typical terrestrial birds from the Pliocene localities of Khirgis Nuur II, Chono Kharnakh, Zavkhan, and Shargyn Gobi in western Mongolia. Two papers by R. Barsbold deal with carnivorous dinosaurs of the Late Cretaceous of Mongolia. One presents new information on the gigantic and enigmatic dinosaur Therizinosaurus; the other, co-authored with S. M. Kurzanov, describes the skull of a new genus and species of tyrannosaurid, Alioramus remotus. In Barsbold's paper on the evolution and systematics of Late Mesozoic carnivorous dinosaurs, new suborders are proposed: Oviraptorosauria, Ornithomimosauria, and Deinocheirosauria. The author argues that carnivorous dinosaurs evolved along several lineages during the Jurassic and Cretaceous periods and gave rise by the end of the Cretaceous to a number of highly specialized groups worthy of subordinal rank. A. G. Ponomarenko and Yu. A. Popov provide an overview of twenty fossil insect localities of Permian, Jurassic, and Early Cretaceous age within Mongolia. The paper by E. S. Stankevich and N. Hand presents for the first time an ostracod assemblage from the Upper Cretaceous Barun Goyot Formation of the South Gobi region. In a pioneering study, O. S. Vyalov, in collaboration with I. D. Sukacheva, proposed a formal classification of fossil caddisfly cases. Five new form-genera are described, and previously known forms are reviewed. Fossil caddisfly cases are of particular importance for the stratigraphy of the Cretaceous of Transbaikalia and Mongolia, as well as the Mesozoic and Cenozoic deposits of the Far East. In a review paper, V. F. Shuvalov and E. K. Trusova compared geological sections from twelve Late Jurassic and Early Cretaceous conchostracan localities and distinguished seven successive conchostracan assemblages within Upper Jurassic-Lower Cretaceous sections. D. Badamgarav and V. Yu. Reshetov reported geological and faunal data from Khaychin Ula V, a newly discovered Oligocene mammal locality in the South Gobi region, found in 1973. A brief multi-authored paper by N. G. Liskun, E. V. Devyatkina, and colleagues presents for the first time the results of studies on diatom and ostracod assemblages from Quaternary lacustrine deposits of the Darkhat Depression in northern Mongolia. G. M. Brattseva and I. M. Novodvorskaya provide information on Early Cretaceous pollen and spores discovered at the Builiastuin-Khuduk and Anda-Khuduk localities, together with data concerning their taphonomy. The second section of the volume addresses problems of biostratigraphy and paleontology of Mongolia's marine Paleozoic deposits, ranging from the Early Cambrian to the Permian. Papers dealing with Early Cambrian faunas are authored by Yu. I. Voronin and N. A. Drozdova, V. A. Sysoev, and Yu. L. Pelman. The first of these papers lists the composition of the oldest known archaeocyathid-algal assemblage (Tommotian Stage) in Mongolia and establishes the boundary between the Vendian System and the Lower Cambrian. V. A. Sysoev described a distinctive form of trace fossil activity discovered for the first time in the Lower Cambrian. Yu. L. Pelman's paper describes brachiopods found for the first time in Mongolia's Lower Cambrian as well as characteristics of a unique molluscan group, the Hyolitha. T. T. Sharkova's contribution is devoted to tabulate corals from the Silurian of the Gobi Altai region in southern Mongolia. O. B. Bondarenko and L. M. Ulitnikov summarize many years of research on Paleozoic corals in Mongolia and examine their geological and geographical distribution. A paper by L. M. Ulitnikov, L. N. Bolshakova, and G. V. Konavnyuk analyzes the taxonomic composition and stratigraphic distribution of Paleozoic bryozoans, stromatoporoids, and related reef-forming organisms from the Gobi Altai, enabling the authors to draw conclusions important for the stratigraphy of Mongolia's Lower and Middle Paleozoic. The paper on Devonian brachiopods was authored by R. E. Alekseeva and B. Mendbat, while the study of Early Carboniferous brachiopods was prepared by I. N. Manankov and E. E. Pavlova. This collection substantially expands knowledge of Mongolia's fossil flora and fauna and its biostratigraphy. It contains numerous new descriptions of fossil organisms and presents new geological information concerning fossil localities. Consequently, this volume of the Joint Soviet-Mongolian Biological Expedition is of value not only to paleontologists and geologists, but also to all researchers interested in the geological history, paleontology, and paleogeography of Central Asia.