in two main clades, but did not recover the western and eastern groups of O. couesi as separate clades. In addition, Adh1-I2 placed the Costa Rican population within the marsh rice rat clade and placed some western O. couesi specimens closer to the marsh rice rat than to the O. couesi group. The combined dataset supported the western and eastern clades within O. couesi and placed the Costa Rican population marginally closer to the marsh rice rat than to O. couesi. Using the genetic species concept, the authors suggested that the four groups they found within O. couesi should be recognized as distinct species. If this suggestion is followed, the eastern subclade would retain the name Oryzomys couesi, the western group would be named Oryzomys mexicanus, and the appropriate names for the Panamanian and Costa Rican species remain unclear. Western Mexico to El Salvador Populations of Oryzomys couesi from Jalisco, western Mexico, east to El Salvador form a single Cytb clade, which Hanson and colleagues proposed to recognize as the species Oryzomys mexicanus. These animals differ by 4.4% from Oryzomys couesi in the strict sense, which occurs to the north and east, are separated by mountain ranges from the latter, harbor different species of hantavirus, and according to Merriam (1901) have more robust skulls, with larger molars, stronger zygomatic arches (cheekbones), and better developed ridges along the margins of the interorbital region of the skull (between the eyes). Within the "Oryzomys mexicanus" clade, Cytb sequence differences average 2.06% and western (Jalisco to Oaxaca) and eastern (Chiapas and El Salvador) groups form distinct subclades; Hanson and colleagues recognized these as different subspecies, mexicanus in the west and zygomaticus in the east. As defined by Carleton and Arroyo-Cabrales in 2009, the subspecies Oryzomys couesi mexicanus occurs along the Pacific coast from central Sonora to southeastern Oaxaca and inland along rivers into central Michoacán, southern Morelos, southern Puebla, and northwestern Oaxaca. It usually lives below altitude, but has been found at in Jalisco. This distributional pattern is similar to that of other western Mexican rodents such as Sigmodon mascotensis, Hodomys alleni, Peromyscus perfulvus, and Osgoodomys banderanus and has been recognized as a distinct biogeographic zone in some reviews. O. c. mexicanus occurs close to three other Oryzomys species—O. albiventer, O. peninsulae, and O. nelsoni—which are larger and different in some proportions and details of coloration. Joel Asaph Allen first described Oryzomys mexicanus as