The retromastoid (“mastoid”) foramina are most often two in number—a
larger and a smaller—one of which transmits a vein from the transverse
sinus within to the cutaneous occipital vein on the outside of the
skull wall, and the other a smaller branch of the occipital artery.
Like the parietal foramina, they show considerable individual and
groupal variation in both number and size. It is not uncommon in some
Indian crania to find one of these canals to be of very appreciable
diameter (up to 4 mm.). While in the Munsee skulls they appear almost
generally two on each side, they are in no instance above moderate
size, and in several cases are quite minute. Thus in these specimens
the retromastoid foramina stand in harmonious rather than compensatory
relation with the small or even absent parietal foramina.