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Seventy-one late Middle and Late Ordovician trilobite species belonging to 50 genera and subgenera, 24 families and 7 orders are described from northwestern Tarim, Xinjiang. Of these, 18 species and 2 genera (the nileid Paraperaspis and lichid Tairongia) are new. Much new morphological information is provided and ranges of variation are revealed for previously known forms on the basis of the new material. Ten trilobite biofacies developed during the Darriwilian-early Katian in relation to environmental gradients of the platform margin, shallow outer-shelf basin and outer-shelf slope are reviewed herein, taking account of the newly described and revised trilobite faunas. Eustatic changes that took place in the region, including a latest Darriwilian-Sandbian transgression and an early Katian regression, are suggested by the shoreward or seaward shifts and the vertical replacement of trilobite associations. Trilobites exhibit close biogeographic connections with the coeval faunas recorded in other terranes of peri-Gondwana, especially in South China, North China and Central Asia. The occurrence of some early Darriwilian genera that were previously considered as typical of either Laurentian (e.g. Nanillaenus, Kawina and Xystocrania) or Baltoscandian (Parillaenus, Panderia and Hemisphaerocoryphe) faunas indicate that faunal exchanges between peri-Gondwana, Laurentia and Baltoscandia may have begun much earlier than the late Katian when the Ordovician faunal provinces finally broke down.