Sr, Mg, and aragonite percentages were examined in modern and fossil shells of Tegula funebralis, Olivella biplicata, and Fissurella volcano. Statistical tests were used to determine whether differences existed between modern and fossil locality means and variances with respect to the 3 measures listed above. Sr was relatively stable with time in the shell material examined, while Mg was mobile; aragonite percentages also appeared to be stable during the time period examined. Mg in shell material of the same species at 2 different locations of the same age reacted differently to postdepositional change.