Ten species of spiny rats of the genus Trinomys are currently recognized in eastern Brazil. Although most taxa known in the genus Trinomys have been characterized on the basis of craniodental, pelage, and bacular traits, data on chromosomal morphology are available for only four taxa, two from the State of Bahia, one from the State of São Paulo, and one from the State of Espírito Santo. Here we describe the normal chromosomal complement for three additional taxa, namely, Trinomys gratiosus bonafidei (Fazenda Boa Fé, Teresópolis, Rio de Janeiro, type-locality), Trinomys eliasi (Barra de Maricá, Maricá, Rio de Janeiro, type- locality), and Trinomys dimidiatus (Rio Bonito, Rio de Janeiro, and Ubatuba, São Paulo). The specimens analyzed here from the locality of Ubatuba were identified on the basis of craniodental, pelage, and bacular traits as T. dimidiatus, extending the known range of this species at least 100km south into the State of São Paulo. The three taxa for which chromosomal data are presented differ in their diploid and fundamental numbers and, therefore, can be diagnosed on the basis of their karyotypes. The significance and implications of chromosome numbers and morphology as diagnostic markers are evaluated in the framework of the molecular phylogenetic relationships and of the data on geographic distribution currently available for the genus Trinomys.
