intense in color than in the marsh rice rat. The snout ends bluntly and the moderately large eyes show reddish eyeshine. The small ears are black on the outside and the inside is covered with short, gray to buff or red hairs. The long tail is dark brown above and white to light brown below. The feet are long and stout. On the forefeet, the ungual tufts (tufts of hair on the digits) are present. Many of the pads on the hindfeet are reduced, as are the ungual tufts, and small interdigital webs may be present in at least some specimens. Some of these traits are common adaptations to life in the water in oryzomyines. As in most other oryzomyines, females have eight mammae. Head and body length is , tail length is , hindfoot length is , ear length is , and body mass is . Studies in Texas and El Salvador found that males are slightly larger than females. The stomach has the characteristic pattern of sigmodontines (unilocular-hemiglandular): it is not split in two chambers by an incisura angularis and the front part (antrum) is covered by a glandular epithelium. The gall bladder is absent, a synapomorphy (shared-derived character) of Oryzomyini. The karyotype includes 56 chromosomes and a fundamental number of 56 autosomal arms (2n = 56, FNa = 56). The autosomes include 26 pairs of acrocentric chromosomes, with a long and a very short arm, and one medium-sized submetacentric pair, with one arm shorter than the other. The X chromosome is either acrocentric, with a long and a short arm, or subtelocentric, with a long and a vestigial arm. The form of the sex chromosomes has been used to distinguish the marsh rice rat from Oryzomys couesi, but there are no consistent differences between the two. As is characteristic of Sigmodontinae, Oryzomys couesi has a complex penis, with the baculum (penis bone) ending in three cartilaginous digits at its tip. The outer surface of the penis is mostly covered by small spines, but there is a broad band of nonspinous tissue. The papilla (nipple-like projection) on the dorsal (upper) side of the penis is covered with small spines, a character Oryzomys couesi shares only with Oligoryzomys and the marsh rice rat among oryzomyines examined. On the urethral process, located in the crater at the end of the penis, a fleshy process (the subapical lobule) is present; it