1. Normal variations in the elementary composition of cortical bone from sixteen different adult vertebrates were studied. 2. Variations in the mineral (ash) and water content of bone exhibited what could be a necessary reciprocal relationship. 3. The mineral components, calcium, phosphorus and magnesium were positively correlated with hexosamine (ground substance) and negatively correlated with OH-proline (collagen). 4. Although positively correlated, the variations in total CO2, sodium and citric acid were much less specific. 5. All specimens exhibited a Ca/P molar ratio of 1.67 or larger depending on the amount of CaCO3 present. 6. These differences in bone composition undoubtedly have physiological significance and their further elucidation is necessary for the valid interpretation of experimental data in all areas of mineral metabolism.