Abstract: A new species of Kherem, K. asiatica, and three new forms of Prosciurus (P. mongoliensis sp. nov., P. cf. P. mongoliensis and P. pisinnus sp. nov.) from Oligocene of Mongolia are described here. Kherem asiatica, known from Shand Member (late early Oligocene-?late Oligocene) of Mongolia , is characterized by : cheek teeth larger and wider than those of K. hsandgoliensis; m1~3 with subequally high metaconid and large hypoconid , with more distinct entolophid and entoconid separated from posterolophid by shallow groove. Kherem is referred to the North American subfamily Cedromurinae. It might be derived from an Oligospermophilus-like form. The three genera so far described from Asia (Sciurus, Oligosciurus and Kherem) may represent three different lineages, indicating the Oligocene diversity of the sciurids in Asia. Prosciurus mongoliensis is known from Shand and Tatal Members of the Hsanda Gol Formation, Oligocene, in Mongolia. Its main features are: brachydont cheek teeth have mesostylid connected with metastylid crest, weak mesoconid, and weak or no ectomesolophid; anteroconid and anterior cingulum are absent on p4; an accessory crest extends from ectolophid to hypolophid on m1; metalophid II is well developed on m2～3; hypolophid bends posteriorly to join with posterolophid on p4～m1 , but straight to join with ectolophid on m2～3. Prosciurus cf. P. mongoliensis from Shand Member is different from P. mongoliensis in having smaller cheek teeth in size, p4 having distinct ectomesolophid , p4 and m1 having gently curved hypolophid , m1 lacking accessory crest from ectolophid to hypolophid , and m2～3 having shorter metalophid II. Prosciurus pisinnus, collected from Tatal Member, is small in size. Its cheek teeth are brachydont with distinct cingulum. The metalophid II is short, and the hypolophid is transverse, low, and joins with ectolophid. The mesoconid and ectomesolophid are well developed, and the hypoconid extends more anterobuccally. The protoconid of p4 has more developed anterior arm. The new material of the Mongolian Aplodontidae not only has confirmed the early existence of Prosciurus in Asia, but also shows that Prosciurus experienced considerable diversity during the Oligocene in Asia. Both the sciurids and aplodontids found from Oligocene of Asia have closer relationships with those of North America than with Europe.

摘要： 蒙古中部地区的三达河组(Hsanda Gol Formation) 是亚洲渐新世陆相地层的经典地层之一。三达河组的沉积岩层现被分成两个岩性段:上部早渐新世晚期- ? 晚渐新世山地段(Shand Member) 和下部早渐新世早期塔塔尔段(Tatal Member) ;两岩段间有时夹有Khar Khoroo玄武岩层(Bryant and McKenna , 1995 ; Meng and McKenna,1998)。该玄武岩层的时代用40Ar/ 39Ar 法被确定为距今31. 5 Ma (HÊck