New species of the Spiny Mouse genus (Cricetidae, Sigmodontinae) from northwestern Ecuador.

is a genus of small spiny or bristly sigmodontine rodents that are common components of mammalian faunas in multiple biomes on Central and South America. Recent studies on this group have demonstrated that there is cryptic diversity yet to be discovered within currently recognized species that have not received comprehensive revisions, as well as in areas that have been overlooked. Here we ratify this assertion by describing a new species previously misidentified as the Narrow-footed Spiny Mouse () from the Chocó biogeographic region in northwestern Ecuador, Brito & Tinoco, Distinctiveness of this entity is supported by the combination of the following morphological characters: small size (head-body length 65-85 mm); long tail (69-126% longer than head-body length); pale buff-colored but gray-based belly fur; white throat; hypothenar pad usually absent; long nasals; and a condylar process higher than the coronoid process. Likewise genetic distance analyses and phylogenetic reconstructions based on cytochrome-b (Cytb) sequence data indicate a clear divergence from typical populations of , and a sister relationship between them. The results presented here increase the diversity of to 24 species, placing it among the most diverse genera within the sigmodontine rodents.