 In the skull , the rostrum ( front part ) is long and robust . The nasal bones are short , not extending further back than the lacrimals , and the premaxillaries extend about as far back as the nasals . The zygomatic notch , an extension at the front of the zygomatic plate , is present . The plate 's back margin is level with the front of the first upper molar . A strong jugal bone is present in the zygomatic arch ( cheekbone ) , so that the maxillary and squamosal bones , which form the front and back parts of the arch , respectively , do not overlap when seen from the side . The narrowest part of the interorbital region ( located between the eyes ) is to the front and the region 's margins exhibit strong beading . Various crests develop on the rounded braincase , especially in old animals . The parietal bones form part of the roof of the braincase and , unlike in some other rice rats , also extend to the sides of the braincase . The interparietal bone at the back of the braincase is narrow and wedge @-@ shaped , so that the parietal and occipital bones meet extensively . 
