Two pelecypods and a bryozoan, all with relatively long stratigraphic ranges, were examined for Ca/Mg ratio with the following results: Anomia sp. went from 48.1:1 to 44.9:1 from Miocene to upper Pleistocene; Panope sp. went from 180:1 to 136:1 from upper Miocene to Recent; and Langenipora sp. went from 38.7:1 to 17.31:1 from lower Pleistocene to Recent. The question is what has happened to the Mg? The MgO content of the Joana Limestone (Mississippian) of Nevada, where it is largely composed of crinoid stems, ranges from 0.30 to 1.9%. The MgO content of modern crinoids ranges from 2.49 to 5.13%. Selective leaching of MgO is discounted because it is generally CaCO 3 which is removed. Alteration of CaMg(CO 3 ) 2 during diagenesis and perhaps the precipitation of CaCO 3 in the porous crinoidal plates are also possible answers for the change in Ca/Mg ratio. There is disagreement as to whether Mg can be lost directly to the sea. The writer placed dead skeletons of Strongylocentrotus and Mytilus sp. in a beaker with sea water which was changed every 3 days. After 4 months there was no change in Ca/Mg ratio.