is absent in all other oryzomyines with studied penes except the marsh rice rat and Holochilus brasiliensis. Skull The nasal and premaxillary bones do not extend back beyond the point where the lacrimal, frontal, and maxillary bones meet. The zygomatic plate, the flattened front part of the zygomatic arch, is broad and develops a notch at its front end. The plate's back margin is located before the first upper molar. The jugal bone, part of the zygomatic arch, is reduced, as usual in oryzomyines. The sphenopalatine foramen, a foramen (opening) at the side of the skull above the molars, is small; it is much larger in the marsh rice rat. The narrowest part of the interorbital region is towards the front and the edges are lined by prominent shelves. The parietal bones extend to the sides of the braincase. The interparietal bone is narrow and wedge-shaped, so that the parietal and squamosal bones meet extensively. The incisive foramina, openings in the front part of the palate, reach backward between the molars. The palate is long, extending substantially beyond the third molars, the usual condition in oryzomyines. The back part, near the third molars, is usually perforated by prominent posterolateral palatal pits, which are recessed into fossae (depressions). Sphenopalatine vacuities are usually absent, but have been reported in some populations. There is no alisphenoid strut, an extension of the alisphenoid bone that in some oryzomyines separates two foramina in the skull. The condition of the arteries in the head is highly derived. The subsquamosal fenestra, an opening in the back part of the skull determined by the shape of the squamosal bone, is present. The squamosal lacks a suspensory process that contacts the tegmen tympani, the roof of the tympanic cavity, a defining character of oryzomyines. There are some openings in the mastoid bone. In the mandible (lower jaw), the mental foramen, an opening just before the first molar, opens sidewards, not upwards as in a few other oryzomyines. The upper and lower masseteric ridges, which anchor some of the chewing muscles, join at a point below the first molar and do not extend forward beyond that point. The capsular process, a raising of the bone of the back of the mandible that houses the back end of the incisor, is large. Teeth The dental formula is (one upper and one lower incisor and three upper and three lower molars on