Limbus of fossa ovalis

Overview
The limbus of fossa ovalis (annulus ovalis) is the prominent oval margin of the fossa ovalis.

It is most distinct above and at the sides of the fossa; below, it is deficient.

A small slit-like valvular opening is occasionally found, at the upper margin of the fossa, leading upward beneath the limbus, into the left atrium; it is the remains of the fetal aperture between the two atria.