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Range DescriptionSaimiri ustus occurs south of the Rio Amazonas in Brazil. It has more restricted range than was supposed by Hershkovitz (1984), who placed it east of the Rio Purus south as far as the region of the rios Sepatini and Guapor, east to the Rios Xingu or Iriri. J. S. Silva Jr. (pers. comm.) believes it extends east of the Rio Purus to the riosTapajs and Juruena, with its range including a belt along the southern margin of the Rio Solimes along the lower reaches of the southern tributaries, including the rios Coar and Tef to the east (left bank) of the Rio Juru. It extends south to the upper reaches of the Rio Guapor and the headwaters of the Juruena. It is replaced by Saimiri sciureus sciureus just south of the Rio Amazonas, east of the Tapajs, on the Ilha Tupinambarama, east to Parintins.