Evidence from previous and present studies indicates that the mineralogical nature of secreted calcareous shells of Foraminifera is principally a genetic characteristic. Mineral content is probably affected, but not determined entirely, by environmental influences. Most genera are wholly calcitic. Relatively few genera are wholly aragonitic and they probably belong to two closely related families. X-ray diffraction studies of 131 genera, having calcareous shells, in 29 families of Recent Foraminifera have shown varying amounts of Mg substitution for Ca in their shells. Certain of the calcitic shells are consistently characterized by low Mg content (0-5 mol % MgCO 3 [or 0-4.3 weight %) and others consistently characterized by high Mg content (10 mol % or higher MgCO 3 [8.6 weight %]); relatively few fall into the intermediate group. A natural division of the genera of Foraminifera, related to Mg content, is indicated. With certain few exceptions, calcitic genera whose shells have high Mg content are restricted to twelve families and those whose shells have low Mg content are restricted to ten other families. Two families seem to belong in an intermediate group. Shells from two families are aragonitic in nature, hence are very low (<1%) in Mg content, a characteristic inherent in aragonitic-shelled organisms. Three families have both high and low Mg shells. That the mineralogy of the shells is not strictly a function of ecology is indicated by the occurrence of both high and low Mg content types under identical environmental conditions. In addition certain genera with Mg content of as high as 10 mol % flourish under arctic conditions, where low temperatures generally result in low Mg content in other calcareous invertebrates. However, deviations to intermediate or low Mg content occur in a few genera in families whose other genera have high Mg. This deviation indicates that phylogenetic position may not be the only factor determining mineral content. Some depression of Mg substitution probably results from the low temperature of arctic or very deep tropical waters. The instances of deviation to high Mg in genera of families whose other genera are low are interpreted as an indication that the genera concerned may be suspect as to their correct taxonomic placement. The determination of Mg content in individual Foraminifera may serve several purposes. First, it may be used as an aid in classification, Second, when the normal Mg content is known for a generic or family group, it may be an aid in determining the original environmental conditions of the living organism. Third, the varying percentages of Mg in carbonate sediments may result from the types of predominant Foraminifera present.