This comprehensive index and summary was compiled by James A. Poupard and Richard B. Thompson. It is an author, subject, location, guide, and calendar to Leidy correspondence at the Academy of Natural Sciences of Philadelphia and the College of Physicians of Philadelphia.

These letters concern various scientific subjects, such as fishes, insects, snakes, mice, fossils, and mosses. Other subjects include learned institutions, notably the Smithsonian Institution and the Academy of Natural Sciences of Philadelphia, and men of science. Correspondents include S. S. Haldeman, Walter J. Hoffman, Spencer F. Baird, Edward D. Cope, and Leo Lesquereux.