PREFACE 



The present contribution to the local flora is intended as an introduction to more 

 extended research. 



The study is of advantage in relation to the life history of aquatic animals, the deter- 

 mination of ocean currents, as proved by polar discoveries, the investigation of geological 

 strata where other fossil forms are absent, and the analysis of water supply; and, when we 

 consider the universal distribution of diatomacese in the earth, the water and even in the 

 air and the enormous deposits formed in past ages and still forming, we are able to realize 

 the importance of a knowledge of these complicated forms and their function of purification. 



The absence of descriptive works of reference in available form in this country, the 

 polyglot confusion of authorities abroad and the amount of time, patience and skill re- 

 quired in obtaining, preparing and examining specimens, render the study one of difficulty. 



The bibliography is omitted, as it is understood by those who possess the works of 

 reference, and but few synonyms are given, having but little, except historical, value, espe- 

 cially when it is considered that modern investigators have no access to many of the 

 earlier collections, when any of these exist. 



So far as the marine forms are concerned, it is probable that nearly all occurring 

 north of Florida are here included, and the fresh-water species described represent a large 

 proportion of those found east of the Alleghanies. All of the figures are drawn to the 

 same scale, a magnification of eight hundred diameters, from specimens in my possession, 

 nearly all of which were found in or near Philadelphia. 



If the work is of any value in inducing further investigation, I hope, in the words 

 of Julien Deby, that "those who follow my advice will find in the study of these wonderful 

 little organisms as much pleasure as I myself have found." 



THE AUTHOR. 



