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P REVIOUS experiments undertaken at this Station have established the value of calcium carbonate as a source of calcium in the diet of growing chicks (1929) and laying hens (1928), also (1923) that highgrade limestone and oyster shell gave practically the same results, as indicated by the growth of chicks and the eggs produced by hens. Halpin and Hayes (1925) reported greater egg production with oyster shell than with limestone which contained a relatively high percentage of magnesium.In our early experiments we found the limestones used as mineral supplements in our chicken experiment contained from 1 to 30 per cent of magnesium carbonate and that the best results were obtained when the percentage of magnesium carbonate in the limestone was small.Hart and his co-workers (1930) have confirmed the work of Bethke and his associates (1929) in showing that when the calcium content of a ration fed to growing chicks varies between .61 and 1.13 per cent, and .3 per cent of phosphorus is present, the optimum Ca to P ratio lies between 2:1 and 4:1, in the presence of vitamin D. Hart and his co-workers showed also that the optimum ratio and quantities of calcium and phosphorus change with the quantity of vitamin D in the diet.Wheeler (1919) has presented experimental results which showed that rations deficient in calcium but carrying abundant magnesium, produced a noticeable shortage of calcium and mineral matter in the bones of chickens. He further showed that magnesium does not serve as a substitute for calcium in egg shell formation.Huffman and his co-workers (1930), experimenting with dairy cows on a low-calcium, high-phosphorus diet, showed that the addition of 3 to 5 per cent of magnesium carbonate produced a clean-cut improvement in the condition of the animal which was coincident with the alteration of the blood picture. Park (1926), andElmslie andSteenbock (1929) have obtained results which do not agree with the results obtained by Huffman. recently working with chickens, has produced evidence to show that when an optimum supply of vitamin D is provided, the calcium requirement of the growing chick approaches .66 per cent as a minimum level and that practical mashes contain about 1.2 per cent calcium. He also found that the phosphorus requirement was about .5 per cent of the ration and that practical rations always contained more. He placed the calcium-phos-