< O O l O g i C d j h m d gst heLinnean Society (l999), 12.5: 415-462. With 6 fiLgures Article ID: zjls, 1997.0165, available online at http://M.Mnv.idealibrary.rom on I Df bl @ a A new species of Oryzomys (Rodentia, Sigmondontinae) fiom southeast Brazil, with comments on the classification of the 0. cupito species group A new species of Olyzomys is described for the coastal Atlantic region of Brazil. Oryzomys seuanezi sp. nov. was collected in eight localities throughout the Atlantic Forest from the south of the State of Bahia to the State of the Rio de Janeiro, mostly in lowlands. Belonging to the 0. capito species group, 0. seuanezi differs from the other species by its karyotype equal 2n=48, FN=60. A morphometric analysis of populations of some species of the group indicates a split between the samples of the Atlantic Forest and those from 0. cupito from Cerrados and Amazon in relation to size, the former being larger. The population from Amazonia and from the Cerrados belongs to the same species, 0. goeldii being a synonym of 0. cupito. 0. seuanezi sp. nov. is the sister taxon of 0. onkcus, forming an Atlantic Forest clade. The close relationship of this group and some Peruvian populations of 0. cupito, probably a new species, is postulated. 0 1999 The Liiinean Socictv of London ADDITIONAL KEY WORDS:-karyosystematics ~ biogeography --morphometry ~ distributionphylogeny.