Cor triplici praeditum ventriculo
This developmental defect of the heart looks like it could be a dissected ventricular septal defect, where there's a hole in the septum, causing mixing between the left and right ventricles of the heart. I don't know of any defect that results in the loss of formation of the atria, but who knows? The Law of Large Numbers means that you're gonna have some pretty funky hearts forming out there...
Specilegium Anatomicum nec non Anthropogeniae Ichnographiam.Theodori Kerckringii, 1717.