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838 the close of the year 1849, under the advice and with the co-operation of Mr. Sullivant, he made a tour of exploration among the mountains of the Southern States, for the collection of plant-specimens, and secured a great variety of plants, which found a ready sale among scientific students. He was particularly successful in the collection of mosses. The preparation of the specimens, their determination and distribution, gave him employment for two years, and resulted in one of the most valuable contributions to American bryology—the "Musci Americani Exsiccati," by W. S. Sullivant and L. Lesquereux. The expense of preparation and publication of this work was defrayed by Mr. Sullivant, who allowed his colleague the benefit of the sales. Using that author's library and herbarium—now the property of Harvard College—for their common studies, Lesquereux lent most valuable assistance to the preparation of Mr. Sullivant's works on the mosses of the Wilkes' South Pacific Exploring Expedition, Whipple's Pacific Railroad Exploration, and the "Icones Muscorum." The publication of Brongniart's "Prodrome," and the commencement of the "Histoire des Végétaux Fossils," in 1828, laid the solid basis upon which the science of paleobotany has been erected. Lesquereux began to write in 1845, and his studies in America have been directed especially in the line of fossil botany. His most valuable researches, beginning in 1850, lay in the study of the coal formations of Ohio, Pennsylvania, Illinois, Kentucky, and Arkansas, and his reports appear in the geological surveys of all these States. Particularly important are his studies of the coal flora of Pennsylvania, published in the report of H. D. Rogers in 1858, together with a "Catalogue of the Fossil Plants which have been named or described from the Coal-Measures of North America." Lesquereux also worked up the coal flora in the second geological survey of Pennsylvania. The fruit of this labor was two volumes of text and an atlas, published in 1880—the most important work on carboniferous plants that has been produced in America. Geological work, especially researches on fossil botany, in connection with the United States Geological Surveys of the Territories, began in 1868 to absorb his attention. He was employed to work up the collections of Dr. F. V. Hayden's surveys of the Territories, and important papers on the subject appeared in the annual reports of the surveys from 1870 to 1874 inclusive. Lesquereux was frequently called to Cambridge to determine the specimens of fossil plants in Professor Agassiz's museum, where he was a guest in the naturalist's household for weeks and months at a time, and his attachment to him grew very strong.

Lesquereux, during his long and industrious life, has contributed twelve important works to the natural history of North America, besides a large number of memoirs on divers subjects, amounting in all to about fifty publications. He is a member or correspondent of more than twenty scientific societies of Europe and America, and was the first elected member of the National Academy of Sciences. The