Ginglymostoma is the genus of nurse shark most often found at Belgrade. I have seen an excellent image of an actual Nebrius tooth from there but it was from the Eocene rather than the Oligocene/Miocene formations. To differentiate the two genus, the center cusp on Ginglymostoma is noticeably much larger than the lateral cusps and stands out as in your tooth. In Nebrius, the center cusp is larger but not as exaggerated as in Ginglymostoma. I suspect that by the late Oligocene, Nebrius no longer existed in the Atlantic.