We describe a new sigmodontine species on the basis of three specimens obtained from a high-altitude locality in the Atlantic forest of eastern Brazil. This new form, a small-bodied pentalophodont with tail longer than head and body, long soft fur, and a brownish ochraceous dorsum, is diagnosed by the presence of an open slit in the suture between the frontal bones in prepared skulls, and by a reduced diploid number of 20 coupled with a relatively high fundamental number of 34. Although the low diploid number suggests a derived sigmodontine, analyses of morphological characters and DNA sequence data (720 bp of the cytochrome-6 gene) point to its placement within the recently described genus Juliomys González, 2000, a taxon regarded as belonging to an old and independent sigmodontine lineage. This finding reinforces current hypotheses of the Atlantic forest domain as an important center of diversification for a primitive sigmodontine stock. It also suggests that at least some surviving lineages, often considered rather ancient and unspeciose relicts, were subject to relatively more recent speciation events.
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