The course of the frontal diploic vein in the mole Talpa and the tree shrew Tupaia is described and compared to the frontal diploic vein of other mammals. The frontal diploic vein in Talpa and Tupaia connects the dorsal sagittal sinus to the veins of the orbit and has an emissary function. In certain other mammals it has a diploic function and may drain towards the orbit (e.g., Orycteropus) or towards the dorsal sagittal sinus (e.g., Didelphis). The frontal diploic vein of these mammals is not homologous to the vein of the human foramen caecum, but to the human frontal diploic vein. The vein of the foramen caecum is a problematic structure: its incidence in embryos and children is not clear.