In non-primates the nasal fossa manifests a posterior intrasphenoidal extension, accommodating the hindermost ethmoturbinals and an accompanying series of paranasal ectoturbinal cavities within the interorbital region. Primitive primates (e.g. Tarsius, Hapale, Saimiri) show phylogenetic loss of these cavities (together with the interorbital region and posterior ethmoturbinals) and the presence of an interorbital septum: a maxillary sinus only is present and the frontal and sphenoid bones are not pneumatized.
In some phylogenetically advanced primates (e.g. Lagothrix, Alouatta) trends of cerebral enlargement and resultant separation of the orbits have induced some degree of reappearance of the lost interorbital territory, and an enhanced but variable pattern of peranasal pneumatization, involving the frontal and the sphenoid. Details of this pattern are described for Tarsius, Saimiri, Hapale, Cebus, Lagothrix and Alouatta, distinction being made between homologous and merely analogous paranasal cavities.