The different geochemical behaviour of the cellular and compact microstructures in Bournonia sp. and Praeradiolites ciryi has been studied through geochemical correlation between both skeletal types. It is the first time that a detailed sclerochronological correlation has been made, laterally following the growth lines from cellular to compact zones, allowing to us to delimit "doublets" in the compact microstructure in lateral continuity with those of the cellular microstructure. This leads to us to a better interpretation of the geochemical record in the rudist shells. It seems interesting to correlate the effects of diagenesis in both skeletal types since the cellular microstructure is thought to have been more accessible to the influence ofdiagenetic fluids, and shows symptoms of clear diagenetic alteration. On the contrary, the biochemical characteristics of both species could have been better preserved in the compact zones of the shells