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Fight species rank taxa are described from Eocene sites in the Gobi Desert, Mongolia. Irdinolophus gen. nov. is erected for Desmatotherium mongoliense Osborn and for Irdinolophus? tuiensis sp. nov. The genus is placed at the stem of the family Deperetellidae. Intraspecific variation in Teleolophus magnus is documented for the first time; Pachylophus is synonymized with Teleolophus; and Deperetella cf. birmanica is recorded far north of its previously known range in Burma. The first known lower teeth of Colodon inceptus support referral of Desmatotherium fissum to Colodon. The stratigraphic range of the hyracodontid Ardymia is extended back from the late Eocene to the early middle Eocene. A range of primitive tapiroids and rhinocerotoids are analysed cladistically to resolve the problematic affinities of the extinct families Deperetellidae and Rhodopagidae. The Deperetellidae are shown to belong to a monophyletic superfamily Tapiroidea, whilst the Rhodopagidae belongs to the Rhinocerotoidea; but is distinct from the Hyracodontidae. Veragromovia, previously synonymized with the tapiroid: Helaletes, is resurrected as a valid genus within the Rhodopagidae. Tsagan Khutel and other sites in the Kholbolchi Formation may be older than Irdinmanhan faunas in Asia, equating with those from the Arshanto Formation of China of probable early Lutetian or latest Ypresian age. The Irdinmanhan Mergen fauna may be slightly older than that from Irdin Manha, because of the primitive aspect of its Lophialetes. (C) 1997 The Linnean Society of London.