For mainly economic or practical reasons the bulk of micropaleontologic effort has been in the study of Foraminifera and Ostracoda. Given impetus by the ease with which micropaleontologic studies assist in oil exploration, many fossil Foraminifera were described and systems of classification erected by J. A. Cushman, F. Chapman, and M. Glaessner. To a lesser but still significant extent, ostracods have been used in oil exploration, and their usefulness is increasing as detailed studies and refinements in classification are made. Other organisms which have been used to a minor extent include Radiolaria, Heliozoa, Chitinozoa, Holothurian sclerites, conodonts, and echinoid fragments. Major problems are the increasing complexities of foraminiferal and ostracod nomenclature, and keeping up with steadily increasing literature. Work done by Russian micropaleontologists has resulted in their language becoming the most important foreign language in the field.