A partial skull of Ardynomys russelli sp. nov. (Rodentia, Cylindrodontidae) is described. This was collected in the late Eocene of Alag Tsab locality in the eastem Gobi Desert, Mongolia. Ardynomys russelli sp. nov. is characterized by small size, brachyodont molars, and retention of P3. It represents the earliest record of the genus Ardynomys MATTHEW & GRANGER, 1925, in Asia. 

    Two families are reviewed (Hyracodontidae and Rhinocerotidae) from the Paleogene of Mongolia. The following taxa are described from the former family:  Triplopus? mergenensis sp. nov., Prohyracodon meridionale CHOW, Prohyracodon? parvus sp. nov., Forstercooperia ergiliinensis GABUNIA & DASHZEVEG, Ardynia praecox MATTHEW & GRANGER, A. mongoliensis (BELIAYEVA), Ardynia sp., Urtinotheríum sp. (or Indricotherium) and Armania asiana GABUNIA & DASHZEVEG. All of them are from the Paleogene of the eastem Gobi Desert The hyracodontid Pataecops parvus RADINSKY is described from a new locality, Menkhen-Teg (Middle Eocene), in the Valley of Lakes.
    In the second family, a new species of Ronzotherium, R. orientale sp. nov. has been recognized from the Oligocene of Ergilin Dzo and Khoer Dzan of the eastem Gobi Desert. Ronzotherium sp. and Allacerops sp. have been described from the Oligocene of Khoer Dzan. The genus Ronzotherium, formerly known from the Oligocene of eastem Europe, has been reliably established in the Oligocene fauna of Mongolia. The genus Symphysorrachis BELIAYEVA, 1954, previously believed to be a junior synonym of Ronzotherium, is here resurrected.
    Descriptions are provided for key localities in the Eocene and Oligocene of Mongolia and the adjacent territories of northern China, containing fossil Hyracodontidae and Rhinocerotidae. In addition, the question of the Eocene-Oligocene boundary in these Mongolian and Chinese sections is discussed. Paleontological evidence has been used to correlate Eocene-Oligocene boundary layers in the eastem Gobi Desert (Mongolia) and Inner Mongolia (China). A brief discussion is provided on the phylogenetic affinities of the genera within the family Hyracodontidae.