The Field Museum’s fossil invertebrate collection started with the purchase of the Ward's Natural Science Establishment collection displayed during the 1893 World's Columbian Exposition in Chicago. The collection grew steadily over the years with the work of A.W. Slocom, S. K. Roy, E. S. Richardson and other Field Museum geologists plus numerous donations from other museums, universities, and the general public. In 1965 the Field Museum acquired the University of Chicago’s Walker Museum fossil invertebrates, which more than doubled the total number of specimens in the collection. Today there are an estimated 2 million specimens divided into ~320,000 specimen lots. The majority of the collection is arranged systematically divided by geologic periods. The remaining collection is organized stratigraphically.