Cor Triatriatum: A malformation of the heart in which the embryonic common PULMONARY VEIN was not incorporated into the LEFT ATRIUM leaving behind a perforated fibromuscular membrane bisecting the left atrium, a three-atrium heart. The opening between the two left atrium sections determines the degree of obstruction to pulmonary venous return, pulmonary venous and pulmonary arterial hypertension.Ebstein Anomaly: A congenital heart defect characterized by downward or apical displacement of the TRICUSPID VALVE, usually with the septal and posterior leaflets being attached to the wall of the RIGHT VENTRICLE. It is characterized by a huge RIGHT ATRIUM and a small and less effective right ventricle.Heart Septal Defects, Atrial: Developmental abnormalities in any portion of the ATRIAL SEPTUM resulting in abnormal communications between the two upper chambers of the heart. Classification of atrial septal defects is based on location of the communication and types of incomplete fusion of atrial septa with the ENDOCARDIAL CUSHIONS in the fetal heart. They include ostium primum, ostium secundum, sinus venosus, and coronary sinus defects.Heart Atria: The chambers of the heart, to which the BLOOD returns from the circulation.