We use a combination of traditional and geometric morphometric plus phylogenetic analyses of mitochondrial cytochrome b sequences to confirm the presence in the Kopetdagh Mts. of northeastern Iran (North Khorasan province) of the Great Balkhan brush-tailed mouse, Calomyscus mystax Kashkarov, 1925 previously regarded as endemic to the Nibit-Dag region of southwestern Turkmenistan. We further compare C. mystax to other brush-tailed mice species in eastern and northern Iran and provide diagnostic external and craniodental characters for each and to examine the affinities with C. grandis with which C. mystax shares a similar karyotype.

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