INTRODUCTION 



Until a recent period, the study of the minute objects which 

 form the subject of this work had been more neglected in 

 this kingdom than almost any other branch of Natural 

 History, and I commenced my researches with the intention 

 of acquiring for myself some fuller and more satisfactory in- 

 formation in regard to disputed points in their history, and 

 also with the hope that I might be able to present to the 

 British Naturalist such a description of our species as seemed 

 necessary towards making the knowledge of them at home 

 keep pace with its advance on the Continent. I soon dis- 

 covered not only that we possessed many species hitherto 

 undescribed, but that various points in their economy, not 

 devoid of interest, remained still unexplained or doubtful ; 

 and, rewarded beyond my expectations, I hastened to com- 

 municate the result of my investigations in a series of papers 

 to the Botanical Society of Edinburgh. As these memoirs 

 have received the gratifying approbation of distinguished 

 naturalists, both in this country and abroad, I have been 

 induced, at the solicitations of my friends, to publish as 

 complete a monograph of the British species as the present 

 state of our knowledge will permit. 



The Desmidiese have been for a long time a common terri- 

 tory, claimed both by zoologist and botanist. In consequence, 

 a greater number of persons have devoted themselves to their 

 study, and as they have often entered on the subject with 



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