The status of Otomys orestes dollmani Heller, 1912 (Muridae: Otomyinae), a rodent described from the Mathews Range, central Kenya

—Morphological and morphometric comparisons of Otomys orestes dollmani, known only from Mount Gargues in the Mathews Range, central Kenya, indicate that the taxon is a valid species distinct from other African forms under which it has been previously synonymized (O. irroratus, O. orestes, O. tropicalis). Based also on these comparisons, the morphological recognition and distribution of O. orestes (including thomasi Osgood) are further clarified in relation to O. tropicalis (including elgonis Wroughton) in Kenyan mountains and O. typus proper in Ethiopian highlands; another taxon relegated to junior synonymy within O. typus, O. uzungwensis Lawrence and Loveridge from south-central Tanzania, is resurrected to species. Certain traditional characters used in Otomys taxonomy, in particular molar lamination, demonstrate conservative patterns of variation that complement spatial structure derived from morphometric analyses of craniometric data and that vindicate their continued utility in delimiting species. We argue that uncritical emphasis of polytypic species, applied following the biological species concept during the latter 1900s, has led to chronic underestimation of species diversity of Otomys confined to the Afromontane Biotic Region in eastern Africa, in particular those populations that inhabit afroalpine environments. Vlei or laminate-toothed rats of the genus Otomys (Muroidea: Muridae) constitute a distinctive morphological and phylogenetic radiation indigenous to the savannas and highlands within Subsaharan Africa (Bohmann 1952, Carleton & Musser 1984, Watts & Baverstock 1995, Ducroz et al. 2001, Taylor et al. 2004a). Since the circumscription of the group as a suprageneric taxon (Otomyinae Thomas, 1897), the number of Otomys (sensu lato) considered to be valid species has vacillated greatly, ranging from 30 (Ellerman 1941) to as few as 8 (Petter 1982) but usually around 12–14 (Misonne 1974, Honacki et al. 1982, Corbet & Hill 1980, 1986, 1991; Musser & Carleton 1993). Musser & Carleton (2005) recently acknowledged 23 species. In the most considered revisionary study attempted to date, Bohmann (1952) recognized only 11 species, most of them containing from 3 to 5 well-marked subspecies and the exceptionally polymorphic O. irroratus embracing 23 geographic races. Much of the indecision over specific diversity has involved populations distributed across eastern African mountains north of the Zambezi River—from the Nyika Plateau and Eastern Arc Mountains, through the ranges and volcanoes fronting the Western and Eastern Rift Valleys, to the Ethiopian Highlands. Within this region, *Corresponding author. PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 119(4):477–515. 2006.