each side of the jaws), as usual in muroid rodents. The upper incisors are opisthodont, with the chewing edge located behind the vertical plane of the teeth. The molars are bunodont, with the cusps higher than the connecting crests, and brachydont, low-crowned, as in most other oryzomyines. Many accessory crests, including the mesoloph on the upper molars and the mesolophid on the lower molars, are present, another trait O. couesi shares with most but not all other oryzomyines. The flexi and flexids (valleys between the cusps and crests) at the labial (outer) side of the molars are closed by cingula (ridges). On the first and second upper molars, the flexi do not extend to the midline of the molars. The anterocone, the front cusp of the upper first molar, is not divided in two by an indentation at its front (anteromedian flexus). A crest, the anteroloph, is present behind the labial cuspule. As in most oryzomyines, the upper molars all have one root on the inner (lingual) side and two on the outer (labial) side; in addition, the first upper molar usually has another small labial root. On the first lower molar, the labial and lingual conules of the anteroconid, the frontmost cusp, are separated by an anteromedian fossette. The second lower molar bears a crest, the anterolophid, before the two cusps, the protoconid and metaconid, that form the front edge of the molar in some other oryzomyines. There is a distinct ridge (anterolabial cingulum) at the outer front (anterolabial) edge of the molar, before the protoconid. The third lower molar also bears an anterolophid and an anterolabial cingulum. The first lower molar has large roots at the front and back of the tooth and two smaller ones in between, at the labial and lingual side. The second and third lowers molars have two large roots, one at the front and one at the back. Postcranial skeleton As usual in oryzomyines, there are twelve ribs. The first rib articulates with both the last cervical (neck) and first thoracic (chest) vertebrae, a synapomorphy of the Sigmodontinae. Anapophyses, processes at the back of a vertebra, are absent from the fifth lumbar. Between the second and third caudal vertebrae, hemal arches (small bones) are present with a spinous back border. The entepicondylar foramen is absent, as in all members of the Sigmodontinae; if present, as in some other rodents, this foramen perforates the