From x-ray studies, it is concluded that the avian egg shell is composed of calcium carbonate in the calcite modification. In the main portion (crystalline layer) the calcite occurs in large crystalline areas oriented with the hexagonal axis (17.6 A) inclined at 28 to 16 degrees from the normal of the shell surface. With respect to the other areas, orientation is present over limited areas. The mammilla layer contains crystallites in entirely random orientation.The findings agree with the electron microscope observations by the second author (Heyn, 1936 a and b) according to whom large oriented crystals, spherulites, or dendrites would compose the main layer and small unoriented crystals the mammilla layer.