As viewed from the right atrium, the floor of the fossa ovalis is formed by
Fossa oavlis represents the site of embryonic spectum primum hence from right atrium its seems to be developed from septum primum. BD CHAURASIA'S HUMAN ANATOMY.VOLUME 1. 6TH EDITION.PAGE NO 255
Septum primum
Septum secundum
Endocardial cushions
Tricuspid valve orifice
Thorax