Although much work has been done in connexion with fossil Ostracoda, little attempt seems to have been made to correlate the results arrived at, and, in consequence, much of the value of the work of such writers as Rupert Jones, R. S. Bassler, E. 0. Ulrich, etc., is in danger of being overlooked. The following is an attempt to collect together in an orderly manner the work of these and other writers, and at the conclusion it will be indicated by reference to the classification of the Purbeck Beds in the Vale of Aylesbury, the use which may be made of these minute fossils to settle questions of great geological importance in cases where no other evidence is forthcoming.