Falx cerebri

The falx cerebri (Latin: "scythe of the brain") is an extension of the protective dura mater that projects into the longitudinal fissure that separates the two cerebral hemispheres.

Details from Gray's anatomy
The falx cerebri, so named from its sickle-like form, is a strong, arched process which descends vertically in the longitudinal fissure between the cerebral hemispheres.

It is narrow in front, where it is attached to the crista galli of the ethmoid; and broad behind, where it is connected with the upper surface of the tentorium cerebelli.

Its upper margin is convex, and attached to the inner surface of the skull in the middle line, as far back as the internal occipital protuberance; it contains the superior sagittal sinus. Its lower margin is free and concave, and contains the inferior sagittal sinus.