Eustachian valve(Anat) a crescent-shaped fold of the lining membrane of the heart at the entrance of the vena cava inferior. It directs the blood towards the left auricle in the fetus, but is rudimentary and functionless in the adult.
The Eustachian valve was small, that of Thebesius imperfect.
"Journal of the Proceedings of the Linnean Society - Vol. 3" by Various