MAMMALS FROM THE EARLY MIOCENE OF THE CHOMUTOV BASIN -I. CHALICOTHERIIDAE (MAMMALIA, PERISSODACTYLA).Abstract. The site described is the brown coal open pit mine Tušimice, part Merkur-North, Chomutov Basin in northwestern Bohemia. Its fauna is defined as a distinct level of the mammalian faunas of the Tertiary of the Czech Republic. From this quarry, the most complete and numerous material of Chalicotheriidae from the Early Miocene of Eurasia is described in detail as Phyllotillon schlosseri sp. nov. The species is characterized as a member of the subfamily Schizotheriinae by the sharp cusps of its narrow upper molars, and a clear separation of metaconid and metastylid of the lower molars. The nearly equal strength of protoloph and metaloph in the upper premolars is also markedly different from Chalicotheriinae. Within Miocene Schizotheriinae, the species represents a basal stage of evolution, distinguishable from the Oligocene genera by loss of the hypoconulid of its last lower molars, and the fusion of the first and second phalanges of the second digit to a duplex bone. Eurasian Early Miocene samples of this stage are almost exclusively dental remains. From the North American genera Moropus and Tylocephalonyx, the material under consideration is distinguished by the lingual lengthening, which means progressive molarisation of the upper premolars. In contrast to Moropus, the metacarpals are considerably shorter, combined with several other autopodial characters. The only Eurasian named genus of this stage is Phyllotillon, from the Lower Miocene of Baluchistan. The Bohemian species is distinguished from the type species P. naricus (PILGRIM, 1908) by the more distally projecting mesostyle of the more shortened M3, and the weaker metacone rib of the upper molars. It has not been possible to determine whether differences of proportions in both species are caused by individual variability. The peculiar, somewhat smaller species "Chalicotherium" wetzleri KOWALEVSKY, 1874 is ranged into the genus Moropus because of its evolutionary level. It is clearly different from Phyllotillon schlosseri sp. nov. The differences from the earliest representative of the more modern stage of evolution, Metaschizotherium bavaricum KOENIGSWALD, 1932 are also observed in detail. The high percentage of chalicotheres is a special trait of the fauna, probably caused by ecological conditions. Analysis of the flora may give new insights into the mode of life and nourishment of this mammalian family.n Mammalia, Chalicotheriidae, Early Miocene, Chomutov basin, North-Western Bohemia, Czech Republic.