specimens from this contact zone and found no grounds on which to separate the two species; thus, he reduced O. couesi to a subspecies of the marsh rice rat. Other workers continued this lumping and by 1971 all other species Goldman had placed in the O. palustris group were classified under the marsh rice rat, together with Oryzomys azuerensis from Panama, described as a species in 1937. Additional studies of the palustris–couesi contact zone in Texas published in 1979, using more specimens and characters, indicated that the two species are in fact easily distinguishable there; therefore, O. couesi has since been regarded as a species distinct from the marsh rice rat. Afterward, some of the other forms synonymized under O. couesi or O. palustris were resurrected as separate species—Oryzomys nelsoni from the Marías Islands, western Mexico, and Oryzomys antillarum from Jamaica. In 2009, Michael Carleton and Joaquin Arroyo-Cabrales reviewed western Mexican Oryzomys, reaffirmed the distinctness of O. nelsoni, and reinstated O. peninsulae from the tip of the Baja California Peninsula and O. albiventer from interior Mexico as species. Still, O. couesi included 22 synonyms, and Carleton and Arroyo-Cabrales wrote that further research on O. couesi and related species would certainly result in the recognition of additional species. A 2010 study by Delton Hanson and colleagues used DNA sequence data from the mitochondrial gene cytochrome b (Cytb) and two nuclear markers, exon 1 of the interphotoreceptor retinoid-binding protein gene (Rbp3) and intron 2 of alcohol dehydrogenase gene 1 (Adh1-I2) to study relationships among populations of the marsh rice rat and O. couesi. The Cytb data placed all studied specimens of O. couesi in a clade sister to the marsh rice rat; the mean genetic distance between the two groups was 11.30%, much larger than the distance between sister species in the related genera Melanomys and Nectomys (7.48% and 7.52%, respectively). Within the O. couesi clade, two populations from Panama and Costa Rica were successively basal to the other specimens, which fell into two large subclades—one containing animals from the Pacific seaboard from western Mexico to El Salvador and the other containing rats from the eastern seaboard from Texas to Nicaragua. The Panamanian and Costa Rican populations differed by 6.53% to 11.93% from the others and the western and eastern subclades differed by 4.41% on average. Data from both of the slower-evolving nuclear markers Rbp3 and Adh1-I2 also placed examples of Oryzomys