K. Vena azygos receiving the intercostal veins.
"Surgical Anatomy" by Joseph Maclise
The azygos veins have imperfect valves.
"Man And His Ancestor" by Charles Morris
The Edentates, at any rate the American forms, have a double vena cava posterior and no azygos vein.
"The Cambridge Natural History, Vol X., Mammalia" by Frank Evers Beddard