During the Cenozoic Era, Radiolaria living in tropical waters developed progressively lighter skeletons, apparently as a result of an evolutionary trend. The average skeletal weight of Quaternary Radiolaria is only one-fourth that of Eocene Radiolaria. If the rate of production of these organisms has remained constant, the amount of silica delivered to the ocean floor per unit time by Eocene Radiolaria is four times greater than that being deposited by Quaternary Radiolaria. Species from the various Cenozoic faunas differ in their resistance to solution. Thus, in assemblages from sediment samples, the presence, absence, and abundance of certain species provide a key to the amount of mixing of faunas and the degree of corrosion of assemblages.