Abstract: The Baron Sog Formation, one of the classic Paleogene formations in Asia, was established by the Central Asiatic Expedition of the American Museum of Natural History (AMNH) in 1925 based on beds at the top of the Baron Sog Mesa. The Baron Sog Formation was also applied to some tope beds of other mesas, such as East Mesa, Erden Obo (= Urtyn Obo) and Nom Khong Obo (= Nom Khong Shireh or Holy Mesa) of the Nomogen (= Nom Khong or Naomugen) area in the Erlian Basin, Nei Mongol, China (Osborn ,1929; Berkey et al. ,1929). However, the wide application of this formation to so vast an area has never been seriously tested. While visiting the AMNH in May and June, 2001, invited by Dr. D. R. Tedford of AMNH, the author was able to observe the specimens collected by the Central Asiatic Expedition and read the related parts of the AMNH archives. Having studied and checked the specimens against the original archives, the author proposes that the age and name of the top bed at the Nom Khong Obo are to be corrected. In this paper, some rodent specimens from the top bed on the Nom Khong Obo are described and the age of the Baron Sog Formation is discussed. All the specimens described here were collected by the Central Asiatic Expedition in 1928. The terminology of the cheek teeth and the direction of the measurements follow Wang (2001).