6 INTRODUCTION 



Pennsylvania, a portion of Maryland on the south and extending eastward to New York 

 Bay and Long Island Sound as far as New Rochelle. 



The greater number of fresh-water species described have been obtained from near the 

 city along the Darby, Cram, Ridley and Brandywine Creeks and from various places in 

 New Jersey, including the Pine Barren region of the southern part of the State. Numerous 

 collections have been made in the Schuylkill and the various reservoirs and along the Wissa- 

 hickon, "where an Alpine gorge in miniature of singular loveliness is to be found within the 

 limits of a city." The fossil deposits are from well borings near Camden, N. J., and from 

 excavations in various parts of the city. 



There appears to be no relation between the Miocene beds of the eastern coast and the 

 deposits here described, all of which have been formed later than the glacial period or in an 

 interval between two such periods. Apparently no diatoms grew during the glacial era, 

 at least in sufficient abundance to leave any perceptible traces of their existence. An 

 examination of glacial "flour" and clays from the Catskills shows an entire absence 

 of these forms, and I have never found them in the milky flow from the glaciers of the 

 Alps nor in the constantly muddy streams in certain of our Western States. The opacity 

 of the water produces the same result as the absence of light in the deep lakes of New 

 England, where diatoms are found only on the stalks or roots of water-plants near the 

 shore, while in shallow ponds, such as the small lake near the summit of Mt. Lafayette, 

 the growth is abundant. Certain species will grow wherever there are moisture, light and 

 heat, but the greater number require the presence, in small amounts, of substances pro- 

 duced by the decay of animal and vegetable life. An abundance of diatoms in fresh water 

 is usually an indication of its potability, while their entire absence in shallow water may 

 be due to an excess of bacteria. 



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 spe- 

 cies 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 



