The specimens from which the drawings are made have been collected by the author for many years; in addition to possessing an almost complete library on the subject, he has had the advantage of examining material obtained by the late Mr. Lewis Woolman and numerous slides furnished by a number of friends, including Mr. John A. Shulze, Mr. Frank J. Keeley and Mr. T. Chalkley Palmer, to whom I here take pleasure in expressing my thanks.
The difficulties of the study are well stated by Agardh in the following extract from the preface to his Systema Algarum:
"Because, indeed, in this respect, no one will wonder whether in the distinction of species and reference to synonyms we have, perchance, committed many errors. They have occurred and are bound to occur, partly from the fact that one is not permitted to see the original specimens of all authors; partly, because sometimes even the original specimens of these plants are erroneous; partly, because the figures and descriptions of authors are often lacking and imperfect....
"There is added the difficulty of the study itself of these plants, their submerged habitat, the minuteness of their structure, the rarity of their fruit, the change in the dried { 7}plant, the impossibility of culture, the fallacies of microscopical vision and the chaotic condition of Algology itself to-day."
The words of Agardh, written in 1824, are almost as true to-day. The lack of authentic specimens, which we hope will be remedied in time by the collections of the Smithsonian Institute, numerous incorrectly labelled slides in amateur collections, the imperfections of figures copied and recopied, without regard to relative size or correct references, and the confusion in the attempts to harmonize different descriptions, deter the student at the outset. The remaining difficulties mentioned by Agardh add, however, to the remarkable interest these forms have always had, since no increase in optical perfection of the microscope serves to lessen the mystery of their structure and mode of growth.
The few species of diatoms first discovered were included by Lyngbye, Dillwyn, and others in the genus _Conferva_. In 1824, the species, increased to forty-eight, were separated by Agardh into eight genera distinguished partly by their mode of growth. But little change was made until Heiberg, in 1863, advocated the division into symmetrical and asymmetrical forms. Without entering upon a general review of the later classifications, including Pfitzer's and Petit's divisions according to the number and location of the chromatophores, or the arrangement of Prof. H. L. Smith, because of the presence or absence of a raphe, or that of Mereschkowsky into motile and immotile forms, the modification of all of these methods by Schuett is here adopted, varied in accordance with certain monographs which appear to offer advantage.