An extraordinarily well-preserved skull of the Uintan brontothere genus Metarhinus, supplemented by a second, less complete specimen, permits a detailed description of the cranial anatomy, including aspects that have never before been described. Of particular interest is the structure of the nasal cavity and internal nares, which may be virtually unique among mammals. The normal air passageway, connecting the nasal cavity to the internal nares, is effectively blocked except for tiny apertures and the functional air passageway re-directed over the top of the vomer, which, itself, has an unusual morphology. The functional significance of so unusual a feature is uncertain, but may have to do with the supposed amphibious habits of this genus, or perhaps, was an adaptation enhancing olfaction. The brain of Metarhinus is found to be typical of early brontotheres, being of small size and consisting of a rectilinear succession of parts: forebrain, midbrain, and hindbrain. The cerebrum is small in size and the cerebellum proportionately large.