High-magnesian calcite coronal plates from three specimens of the clypeasteroid echinoid Peronella lesueuri Agassiz, collected live from Shark Bay, Western Australia (113 degrees 30'E, 25 degrees 30'S), were analyzed for magnesium distribution. Electron microprobe spot analyses indicate an average content of 12.8-13.2 mole % MgCO 3 . Although this average agrees with X-ray diffraction and atomic absorption analyses completed on the same specimens, point-to-point compositional variations ranging from 10.3 to 17.0 mole % MgCO 3 were detected in electron microprobe work. Thus, some heterogeneity in Mg distribution is indicated: this heterogeneity is not related obviously to either the crystal fabric or the particular coronal plate analyzed. No mineralogical phase with more than 17.0 mole % MgCO 3 was detected. Coronal plates from 5 Pleistocene specimens (> 30,000 years B.P.) of P. lesueuri obtained from the Carbla Oolite at Shark Bay were also analyzed for magnesium distribution. Of more than 400 electron microprobe spot analyses made, about three quarters consist of values of 3.9 mole % MgCO 3 or less, and clearly represent low-magnesian calcite. There are, however, many additional spot analyses in the range 4.0-17.0 mole % MgCO 3 , with the majority in the range 4.0-9.0 mole % MgCO 3 . These values appear to be derived from minute, Mg-rich zones on the order of 1? micron or less up to 6-7 mu , as determined by beam-scanning photography with the electron microprobe. These zones are interpreted as metastable domains which reflect either no loss (rare), or incomplete loss (relatively common) of Mg from the original skeletal calcite precipitated during the life of the organism, and are believed to indicate inefficiency in the stabilization process by which high-magnesian calcite is eliminated from Peronella lesueuri individuals. The Mg-rich zones suggest, but do not prove, that P. lesueuri calcites have stabilized by loss of magnesium to an aqueous solution while retaining the initial geometry of skeletel elements--the incongruent dissolution described by Land (1967). The Mg-rich zones could be the location of dolomite nucleation at a later stage in diagenesis, as suggested by Schlanger (1957, 1963), for dolomitization of Mg-rich calcareous algae. Two of the Pleistocene specimens contain dolomite as determined by electron microprobe and X-ray diffraction analysis: the dolomite is interpreted to be of detrital origin.