Most commonly, it is a crescentic fold of endocardium arising from the anterior rim of the IVC orifice. The lateral horn of the crescent tends to meet the lower end of the crista terminalis, while the medial horn joins the thebesian valve, a semicircular valvular fold at the orifice of the coronary sinus. At the other extreme, it persists as a mobile, elongated structure projecting several centimeters into the right atrial cavity.
Typically, this is a curved fold of heart tissue arising from the front edge of the IVC opening. The outer part of the curve usually meets the lower end of the terminal ridge, while the inner part attaches to the thebesian valve, a semicircular flap-like structure at the coronary sinus orifice. At the other end, it persists as a flexible, elongated projection extending several centimeters into the right atrium.