what is fossa lunata ?

It is present in the left atrium and is the counterpart of the fossa ovalis of the right atrium. There are four chambers in human heart  Left atrium is one of the four chambers of human heart. Fossa ovalis somewhat sunken structure found as depression in the right atrium of the heart. 
Inside the left atrium fossa lunata is semilunar depression, found on the surface of the atrial septum.
It is actually remains of the depression of foramen ovale that is found in foetal heart . Foramen ovale, in foetal heart allows blood to enter the left atrium from right atruim. This opening foramen ovalis becomes closed at the time of birth. In man the remains of this foramen is present in right atrium as fossa ovalis and in left atrium as fossa lunata.