This classification is made on the plan presented in the "Encyclopedia Britannica," and includes genera and species not included there, especially of North America and particularly the fresh water fish of Pennsylvania and New York states, ca. 1866-1879. Also included are many illustrations of specimens.

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.